Lock Up
 

The symptoms you describe are very familiar. It seems like you have a heat problem. I bet with the door off you will play without problems. My first indication of a problem like that was after putting in a Radeon 9800 Pro 256MB, I would hear a beep and graphics would mess up. I had a 350 Watt power supply running a well stocked home built.

P4 2.66 GHz / 1 Gig PC2100 Mem / ATI Radeon 9800 Pro 256 MB/ 4 hard drives / Plextor CD-RW / Sony DVD DRU 500A DVD-RW -R & +R & CD-RW / Lite-On DVD Player / ATI TV All in wonder & 3 Fans plus USB Scanner and USB Printer.

Bumped the power supply up to an Antec 480 Blue and have not seen a hic-up yet. Runs smooth and COD/MOHAA/BF1942 running full bore at highest settings.

Don't overload the power supply

The Ole Sarge