HD2600XT AGP No DVI signal while playing games

Problem:
When playing 3D games the GPU stops responding and has to be reseted by ATI VPU Recover after about 30min gameplay and after another 30min the GPU looses signal again. (tested games Crysis Demo, Call of Duty 4, Hellgate London)

Troubleshooting:
Installed GPU drivers from scratch (uninstalled previous ones with "Driver Cleaner Pro" from guru3d). I've tried with both the drivers from the included Driver CD CCC 7.7, the new sapphire CCC 7.10 Driver and the latest Omega Driver all with the same problem. My PSU should handle my system without any problem. I couldn't monitor temps with either ATI Tray Tools or RivaTuner so I had to go after the Everest and ATI Overdrive Tool and watch the temp during Auto-Tune. They were ~50c at Idle and ~60c during 96% load, OC:ed the temps were ~60c Idle and ~70c load.
I'm starting to think that I've got a bad card.

System Specs:
CPU: Inter P4 2.4GHz Northwood OC:ed to 3.0GHz
Motherboard: ASUS P4P800
RAM: 1GB Corsair 3200 DDR (2 matched 512MB Sticks)
PSU: Corsair VX 450W 33A on 12V rail
GPU: HD2600XT AGP
OS: WinXP Pro SP2 32bit