Intermittent problem

I' running a Asus P5GC-MX with a Intel Dual core E2200 2.2 GHz, normally a XFX Nvidia GTX 260 with 4 GB of ram on Vista. Normally i don't have any problems running everyday normal programs like Firefox,word, or anything else but when i go to anything that has 3d in it like a video game the screen goes on the fritz occasionally BSOD'ing, causing the video driver to restart, causing the open program to be close, or making the screen unreadable (looks like an old analog TV when your not tuned into a channel) and the only way to fix this symptom is to completely reboot the computer. It hasn't BSOD'd lately not sure why it hasn't but I've constantly had problems like this since I purchased the computer back in November. I went out and purchased a second different video card(Nvidia GeForce 9800 GT) for diagnostic purposes. Then when i booted up for the first time after installing the drivers i started a game(World of Warcraft) and no less than 5 seconds of running it, it made the screen unreadable so i had to reboot. When rebooted started the game again and it worked fine. Attempted another game(City of Heroes) and first 3 attempts made the video driver restart and close the game and then on the 4th attempt it worked. Seeing as the second video card didnt fix it im not quite sure as to whether its a video or motherboard problem. Anyone have any ideas about these intermittent problems?