Mac Pro 3.1 black screen, loud fans

Hello community,

I have a Mac Pro 3.1, 2008 dual 2.8ghz computer. I bought it cheaply as the original owner claimed it had a problem where it would randomly sleep, and a hard reset was neccesary in order to restart the computer. On turning on the computer myself I soon saw what he meant, all of a sudden, quite randomly the screen would drop out entirely and about 8 seconds later the fans go on full whack. This could happen before the computer boots, or could happen 15 minutes later. Sometimes it would happen more often when say I watched a youtube clip on full size in 720/1080p. This shut down can only be stopped by hard resetting the unit.

Originally i thought it might of been the graphics card (Nvidia 8800 GT) which upon researching had found out there have been issues with this graphics card for other people in the past, however they speak of glitchy graphics, over-heating and 'baking' the card. I tested the temps on the gfx card and they were not abnormally high at all. Infact on monitoring all the temperatures when the computer was on i noticed no abnormality with any temps apart from maybe the north bridge which was max 62 C when idling. But my macbook pro hits 75 C on the CPU when using intensive software (logic pro etc). I also tested the gfx card in both available pcie slots aswell as cleaning out excess dust from the gfx card using compressed air.

The computer has a WD velociraptor 128gb 10k rpm HDD installed. I tested the computer with a 128gb SSD which i know worked. I managed to install snow leopard from a DVD onto it before a crash and the computer worked for maybe 20-25 mins then it blacked out again. Not the hard drive then. Tested all the ram (6gb), moved them around etc, no red dimm lights on either board. Not the ram.

The only main components left really are the power supply and the logic board. I have no red leds on the logic board lighting up, so no obvious failures. And when the computer is on it works a treat.

Does this sound like the PSU is dying/dead? I thought this could be an easy software fix, but am now chasing the hardware train....

Any help would be greatly appreciated and thanks in advance!

Cheers!

Catro