Video screws up on Nvidia Geforce2

I have a PIII 450 mhz with 256mb ram running windows 98SE, directX 8.0 on a gigabyte GA6BXC motherboard. The video card is a Nvidia Geforce2 MX400 64 mb ram with tv out. After I play simcopter for a bit the video turns to about 2 colors or so. You can still see sortof what your doing but you have to go out to the game menu and come in again to restore the video. It also does this on Railroad Tycoon from Microprose. It does not mess up on Flight Simulator. I have tried a different monitorand the computer company replace the video card with a new one. Same thing. The screen will turn red or blue or some other color but you can still see shapes, etc.