Try using an alternative Glide wrapper, e.g. the one from zeckensack.
http://www.zeckensack.de/glide/

Sorry I'm a bit of a noob when it comes to computers, what exactly is a glide wrapper?

King's Quest 8 supports the old 3DFX Glide technology, but modern graphics cards don't. Glide wrappers are programs which emulate 3DFX technology on modern video cards. The game thinks it's in an environment with a 3DFX graphics card. The wrapper takes the game's commands and translates them into nowadays supported OpenGL. The two most compatible glide wrappers for KQ8 are dgVoodoo and Zeckensack's glide wrapper.

Weird that GoG wouldn't have formatted the game so it ran on current computers... But thanks for the clarification, I'll check it out.

GOG made an agreement with Zeus, the guy who wrote nGlide, to include it in their installers for supported games. Problem is, GOG doesn't have a clue that this wrapper is the worst for this game and Zeckensack has been MIA for several years so they can't get in touch with him to get permission to include it.

dgVoodoo 2 is really recommended if you have a DirectX 11 capable video card, since it runs a lot better on newer systems. http://dege.freeweb.hu/

You should probably install a different Glide wrapper than the one that comes with GOG.com's release of KQ8. Below I try to guide you through how to select the Glide wrapper that will be best for you, and then how to disable nGlide (what comes with GOG.com's release) and install the other Glide wrappers.

To find out what DirectX version you are using, go to your start menu and type in "dxdiag" (without the quotes) and press enter (If you are using Windows XP you don't have DirectX 11, but if you are still curious you can press "run" on the start menu and then type in "dxdiag" there). A "DirectX Diagnostic Tool" window should pop up, with a box labeled "System Information" in the "System" tab. Towards the bottom you should see "DirectX Version:" followed with whatever version you have. Use that in deciding which Glide wrapper to use.

If you have a DirectX 11 graphics card, I would highly recommend you use dgVoodoo 2. It takes more effort to get working correctly, but for me at least it works really well.
If you have anything lower than a DirectX 11 graphics card, I would recommend Zeckensack's Glide wrapper. For me it worked really nicely "out of the box", but the framerate died when I had dynamic lighting on.

Anyway, something that I don't know if you'll find anywhere else, how to install the Glide wrappers:

Remove/disable nGlide:
For both dgVooDoo 2 and Zeckensack's, you are going to want to remove, or at least deactivate, the Glide wrapper that comes with GOG.com's release of the game. Here is what you do:

1. Go into your installation directory (probably C:\GOG Games\Kings Quest 7-8\KQ8 unless you changed it)
2a. Delete glide2x.dll and any files starting with nglide
OR...
2b. Rename glide2x.dll to glide2x.old.dll

dgVoodoo 2:

1. Download and extract dgVoodoo 2
2. Go into your installation directory (probably C:\GOG Games\Kings Quest 7-8\KQ8 unless you changed it)
3. Copy the Glide2x.dll from the extracted dgVoodoo 2 archive (it is in the 3Dfx folder) into the KQ8 folder
4. Launch dgVoodooSetup.exe
5. Start playing with the settings under General and Glide. The following settings work for me, but your setup may require different settings (for instance, the following settings have it run in a 4:3 aspect ratio fullscreen on my computer, but on my sister's computer I had to go through the 4:3 resolutions on the Glide tab until I found a few that made it look right).

General:
Appearance: Full Screen; Prefer centered full screen
Miscellanneous: (checked) Keep window aspect ratio

Glide:
3Dfx card: Voodoo Banshee; Onboard RAM = 16 MB
Texturing: Memory size = 4096 kB; Number of TMUs = 1; everything unchecked
Resolution = 1920x1080 (to check what your current resolution is, if you are using Windows 7 or greater, right click the desktop and click Screen resolution. Do not exceed your desktop resolution)
Antialiasing = Off
Miscellanneous: (checked) Enable Glide Gamma Ramp; (checked) Force emulating true PCI access, everything else unchecked

Zeckensack's:

1. Download Zeckensack's Glide wrapper (download link is near the top)
2. Run the installation application, leaving everything checked.

If I recall correctly, you should be able to run the game and everything should work just fine (although as mentioned above some people, myself included, have framerate issues when dynamic lighting is turned on). If, however, it doesn't or you just want to mess with the options available, go to Start > All Programs > Glide wrapper > Configurator.

Some more information:

Also, in case you don't know, I feel I should warn you that you will likely run into areas where your game will lock up during a cutscene. If that happens in one of the common locations the fix here should help you, as all the script files are named: http://www.gog.com/forum/kings_quest_series/kq8_game_wont_give_back_control_after_cutsceen If it happens in an uncommon location, you might be able to find the script file you have to edit by following the suggestions from iondrive here: http://www.gog.com/forum/kings_quest_series/kq8_so_close_to_end_but_now_im_crashing

Just want to try and prevent you from having to do the hours of searching that I did.

Hope this helps!

Great post! This is the kind of concise, methodical info that needs to be stickied in this forum--why Gog sells a Glide-wrapper game without detailed instructions like this in the forum in a sticky thread is beyond me. Perhaps they don't bother because they think "Well, you can always run it in software." That's the wrong attitude. The game is infinitely more enjoyable with a decent Glide wrapper--software really ought not be a KQ8 option for anyone, imo.

I think that possibly some additional detail won't hurt, either. I'm running KQ8 in Win8.1.1x64, and the dgV2 wrapper won't work at all for me, including version 2.31 that was put out just three days ago (!) by Dege, presumably. In speaking with Baggins and others in this forum, it would appear as if the dgV2 versions might be fine with Win8.1x86, but not like x64, as it won't run properly for me--as in it will not open an in-game screen so that I can actually play the game...;) At any rate, Zeckensack's does a fine job for me in KQ8.

Still, the nerd/hacker in me would like to get dgV2 to work...;) It simply refuses my every overture...:D

There's actually no benefit from changing it from the default setting: Voodoo 2, Onboard RAM - 8 MB, Number of TMUs - 2.

Regarding resolution, any other settings than 1280x960 (640x480 in game settings) or 1600x1200 (800x600 in game settings; that means you need a 16:10 monitor with 1920x1200 resolution... no 1080p monitors!) will have distortions in all the interface graphics like the menu, subtitles, and mouse cursor. The 2d graphics have to be processed at 640x480 or 800x600 internally for everything to align correctly, it's simple mathematics and it won't be pretty scaling up to anything else if it's not an even multiple of the original resolution. Not having any problems here, I'm running Windows 8.1 Pro with Media Center x64 and dgVoodoo 2.31 runs just fine. See the attached screenshot; running in windowed mode for illustration.

Well, I changed a lot when I was trying to get it to work, so once it did I just left everything as it was in fear of breaking it again.

Also, with the settings I listed I didn't notice any distortion, I seem to recall the cursor was a little jagged, but that's it. dgVoodoo 2 seems to react significantly different on different computers, though, because using the exact same settings on my sister's computer did cause distortion.

Interesting...I actually had some back & forth with Dege in another forum, and we did some bug hunting, and it turns out there's something in the Catalysts that doesn't work well with his wrapper--he's sent in a bug report to AMD about it....However, I did find one wrapper that worked fine (Zeckensack's)...and while we were talking about that one...nGLide was updated to version 1.03 that now handles KQ8 fine in 8.1x64...so all's well that end's well...;) There are a couple of threads here in the forum where these wrappers were discussed recently!