Broken Power Supply or Graphic Card?

A few weeks ago, my computer started to randomly restart while I play certain video games, such as Left4Dead, GRID, DiRT. Games like these will cause my computer to reset during game play and games like Call of Duty 5, Crysis, and Farcry 2 work without any problems.

I've test my CPU, RAM, PSU, MotherBoard, and Video Card. My conclusion is that it is probably the video card (which is an ATI HIS HD 4870) or the power supply (which is an Ultra X3 800 watt PSU).

I can run the games fine as long as my GPU clock is 500MHz and my memory clock is 760MHz. The default setting originally was 750MHz GPU clock and 900MHz memory clock but running under default setting the computer will reset during the games that I have previously listed. While running under default settings, the GPU temp. never exceeded 60 degrees Celsius. This leads me to believe that the problem has nothing to do with overheating. Under default settings while playing GRID, I am able to enter game play for 1-5 minutes before the computer resets. Whenever I increased the GPU clock up to 775MHz and the memory clock up to 950MHz, GRID crashes while in the game menu and I am unable to make it to game play. This has led me to believe that the problem is in the PSU.

I am going to list what I have already done:

I have checked the CPU for overheating and the steps taken to address issue was to replace heat sync.

I have checked the memory (ram) for corrupt ram & overheating and the steps taken to address issue was to run memtest overnight & cool ram with fan.

I have checked the video card for overheating & corrupt video memory and because it crashed at 52 degrees celc. (which should handle up to 90 degrees celc.) & card runs L4D and GRID when GPU clock is set to 500MHz and memory clock is set to 760MHz it has led me to believe that the video card is functioning properly.

I have checked the PSU for weak power supply and I checked the PSU with an Ultra power supply tester (also shown at http://www.tigerdirect.com/applica [...] 3&CatId=1) which is showing that the PSU is giving off the correct voltages.

I have checked the motherboard for broken PCI-E port and I addressed this issue by switching card to different PCI-E slot and problem still persisted.

Do you know if there is a software application that I can download to check for voltages? I do not own a physical voltage checking meter.