Has anyone had problems with flickering video on BOOH (expansion)? I've toyed around with the video settings but nothing seems to help. At first, I thought maybe it was supposed to be that way (like, it's the opening level, trying to be spooky), but looking at it closely it seems more like it's not refreshing properly and you can see flickering).
I played through the original game off of GOG without any issues.
I'm not super sophisticated technically, but willing to try some things out if anybody has any ideas that might help.

Try forcing vertical / V sync in the options menu.

I take it that is a setting on my video card (not from the game video options)? I have an ATI Radeon HD 4870x2. In its control center, Monitor Properties, Adustments, Synchronization, there is a Horizontal and Vertical adjustment I can increase + or decrease - . I should adjust that?
Sorry for my lack of technical knowledge (and thank you for helping!).

Ahh your using a 4870x2 that may be your problem. You could still try the V sync option can't remember if there was one actually in the game or not but it should be possible to set it for painkiller only to force V sync using your catalyst control panel. Unforutantly I haven't used a ATI card in many years now so I have no clue how you would find to do this.
If that doesn't work then you may need to turn off the second GPU core on your 4870x2 so that its only running one of the graphics processors unfortunalty again I'm not sure how you would go about doing this. Though the options should also be in the ATI control panel. I'm sure if you can't figure it out someone here that uses ATI cards can help you find the options.

No luck, sadly. Thank you for your help Ralackk.
If anyone does have an idea how to force V Synch on this ATI card, let me know your thoughts. I can try moving on to shutting down the second core too if that doesn't work.