I'd do two things.
1. Remove the case cover to sniff out heat issues.
2. Install the latest Nvidea drivers from Nvidia and not Microsoft.
Im having issues with my video card. Awhile back it just started flickering and freezing up on me. So i assumed it was the card itself. I switched to the onboard video and have no problems. Yet when i uninstall the driver for my plug and play card i can then put that card back in and as long as i dont update the card and use the default monitor it does not do any flickering or freezing. So im not exactly sure its the card because as long as i dont update it and use default driver for it, it works fine just without any 3D. By the way its a nvidia Geforce4 mx420.Any suggestions would be appreciated.