Hardware troubles and im running out of people to go to...

[Chro]

Hello! ive been asking around about my hardware problems (pretty sure its my radeon 9800 pro) and hopefully you smart people will be able to help me out.
When ever i play the high-end games such as doom3, Command and Conqure generals or final fantasy 11 my computer will completely freeze up, not right away. but mabee if i play the game for an extended period of time. (so i thought it was over heating) All i get is a humming sound from my speekers and the screen freezes. Its never when i multi-task or use up alot of cpu speed, so i dont think its the chip or mother board.
I tried useing Atitools to turn the speed down to 290/290. (im new to it so i just tried reducing speed by 80-90) ummmm im open to any suggestions if it could be something else or if i should change the clock speed of the video card. Any help is greatly apprechiated, and thanks for you time!

[Zedicus]

turn off fast writes and set yur agp speed to 4x

[ReconCX]

your video card might be overheating, try running 3dmark03 once.. then run it on loops, if it crashes in the loop sometime, that might be it. check to make sure your fan is working on your videocard and that the heatsink hasn't become loose. Run a few CPU burn-in programs and see if that causes crashes too. Finally, check your voltages if your motherboard supports it, log the files. If there are big variance spikes in voltage, it may be that your PSU is failing. Again, be sure to check all cooling aspects of your PC. Keep the case open while running and see if that stabilizes the PC at all.

[Chro]

Alright, i tried the set my agp speed but then it says that it checks to see if it will be harmful and when i reboot its set back off. Ill try the 3dmark03 next, thanks again!