PC crashing on the Furmark 1080 test but runs 720 test fine

[Sparky]

It's a Sapphire HD7950 3GB card that's 3 years old. The PSU is a Thermaltake 750W 80 Plus Gold which is 2 years old. All diagnostic software shows the general line power is where it should be, I know the RAM and CPU - both less than a year old - are fine as well. So it has to be the PSU or the vid card. When I'm playing something less intensive like Civ 5 I can play all night and no problems. When I play something intensive like Wolfenstein The New Order, et al. I can play for a while - times vary depending on grahpics intensity, say 15-60mins, and then the PC will just reset, post, and come back to 'Windows didn't shut down properly, blah blah blah.' After a rest I'll jump into the bios and look at temps, and everything is find for CPU and mobo.

As stated it runs the 720 test fine, though it shows the GPU at around 69C and the fan only about 51%. When I launch the 1080 test it almost instantaneously resets. I've done it 4 times in a row now and it's been the same each time. So, does it sound like the PSU is the culprit? At least it's still under warranty. Unfortunately I don't have a spare or access to one and don't want to buy one to return it and get dinged a restock fee just to test it out. Would a good PC repair place be able to test the PSU for me? Also, if anyone has had to RMA a PSU to Thermaltake, any idea how long it takes?

Thanks!
Sparx

Edit: I just physically looked at the card and only one of the fans is running - had this problem - the fan, not crashing system - with the previous card which I RMA'd. What are the odds? Maybe the PSU has been the culprit all along.

Edit 2: Ok, took PC apart to inspect and clean everything. Finally saw that PSU fan isn't working. Blew everything out with compressed air, reseated everything, dotted the i's, crossed the t's. When the PC boots up I notice the second fan on the card starts to rotate for a second then stops. So I'm guessing not enough juice for it - again this was the problem with the original card too without the crashes however. So I'm almost 99% certain it's the PSU at this point. I guess I'll grab a new one, RMA this one and keep whatever they send me as a spare.

[JeGX]

yes it looks like a PSU issue. If FurMark resets your PC instantaneously, it's often caused by the PSU. Thermaltake has good PSUs but I already had some issues with them. The best are Corsair's. Try the Corsair AX760i. I have the AX860i and it's fantastic PSU. It handles all my graphics cards without problems (some of them can draw up to 500W!) and is very quiet.

Let us know.

[Sparky]

Yeah, so it was - in part at any rate - the PSU. I can run the 1080 tests without it crashing the PC now.

I'm RMA'ing back my Thermaltake so I'll have a spare, but I went with the EVGA Supernova 750 G2 80plus Gold as a replacement. All the reviews say it's amazeballs, so I guess we'll see. As for the vid card, the rear fan still isn't working. So that's twice now - and the last time I ever buy anything from Sapphire, and I recommend everyone else avoid them like the plague as well.

[JeGX]

Never tested EVGA's PSUs but they have good reputation and the Supernova 750 G2 should do the job perfectly.