Did you try windowed mode?
Does it make a difference if you drop the resolution and lower the graphics settings?
Did you check your power settings in Windows? Try high performance and disable power saving modes for the monitor.
If you don't have similarly demanding games to try, you could run 3DMark or something to see if that causes the same problem. If this just happens with D:OS, I don't know what is going one; in general it could be caused by the video card overheating, power supply, or something.