Sry for my english.

I just installed Schleichfahrt and in the missions the game just slows down, runs normal, slows down, ect. also the sound/music stops for quick moment at the same time the game slows down. What causes this? Has 3dfx something to do with it? At this state its just unplayable.

I don't have this game on GOG but does it have software mode also or just hardware/3dfx/glide?

It also has software but I don't know how to deactivate 3dfx for Schleichfahrt so that I could test it.

edit: changed the exe in the dosbox config and now it is running in software mode. The issue is gone so 3dfx was causing the problem. To bad I can't play it with 3dfx but better than nothing.

Isn't there a shortcut for the software version? I know Carmageddon has it.
I tried playing Archimedean Dynasty 3dfx mode a couple of years ago, went back to software because it ran too slow.

When I got my Archimedean Dynasty up and running last year I had a similar sounding issue where the sub would move fast then slow, then fast then slow, giving a chugging feeling to it as if the frame rate was oscillating instead of running steady. This was in 3DFX mode. I resolved it by turning off vsync in the nGlide configurator before running the game.

Problem solved. I tried turning vsync off in the nglide settings but it seems not to work also the splashscreen doesn't go away with off options. BUT I lowered the resolution to 1024x768 and now it works like a charm.

Just remember that 3dfx was capped at 800 * 600 at the time. I don't know how effective the nglide wrapper is so reducing resolution should definitely help

Switching off Vsync did the trick for me, no more stuttering!

That's how it worked for me as well. Turn off "vertical sync" in the "nGlide 1.03 configurator" AND in the NVIDIA Control Panel:

How do you fix the stuttering with the DOSBox SVN-Daum version? It says the V-sync is turned off but the game is still stuttering.