I had a similar problem memory was to blame , start troubleshooting .It could be several causes .If you havent done so run extensive tests on your memory I recomend running Memtest overnight to rule out memory issues. My Asus P4G8X MB by default setting had the AGP voltage set at 1.6v i run mine at 1.5v . There has been some issues in the Video Memory Cache Mode bios settings .Ive tested many cards and my Ti 4600 ultra card ran the best on the UC mode.I tried both settings in the USWC mode 3Dmark 2001se yielded a score of 8000+- and games were very unstable .In the UC Mode i yielded a score of 14500 so thats a big difference.Heat combined with other problems can allways be an issue , make sure you have good ventilation .
Power supply is often overlooked but is the most important item in any sytem . I had a cheap 500 watt Channel Well PS that never put out the needed voltage on all rails.If you have high end hardware you need a (quality)400watt or better PS .
The video card itself may be to blame how old is it ? Does it have a fan on it ? If so make sure its spining , if not Its easy enough to install a fan on it which will help any cards performance.Hope this helps good luck