New rig randomly powering down?

Hey all, just put together a new rig a week or two ago; everything's been working absolutely fine from the get-go, except a few days ago it abruptly powered down. I'm posting now because though I was just hoping it was a one-off, it's just now happened again.

In both instances the computer instantly loses all power (akin to a power cut, though that's obviously not the case), and without any delay whatsoever, "restarts" itself. I say it in that way because it doesn't actually manage to restart, it just powers everything up and loiters on a black screen doing nothing. When I turn it off and back on again, it boots normally.

As for details, the first instance was when i'd been playing Mass Effect for a couple hours (which isn't any stress in this case; will post specs below); I should specify i'd played the same game (and even the same part of the game) before and after that incident with no problems whatsoever, and afterwards I even tabbed out frequently to check my GPU and CPU temps incase overheating was the problem. In every case, ME fails to stress them beyond about a 40c heat (they idle in the high 20s-low 30s). Incase overheating was the problem, I did set up and run furmark for a while on stability test, and it maxed out at 60c, where it loitered with no problem.

Hoping it was just a one-off, I tried to forget about it and moved on, and everything was predictably fine for about another week; I could happily play games, watch HD video and do my work (photoshop, corel painter etc.) for any amount of time without issue.

For the past couple hours i've been watching some standard-def MKVs; about 45 mins ago I started getting on with some work in painter, without any issue, for about 30 mins (was a fresh image, so again, nothing that would remotely stress the computer), when it happened again; the entire thing just lost all power, booting to a blank screen, until I restarted manually.

Incase it's relevant to a diagnosis, I should point out that when I started up again, my RIF file from painter (which I had saved several times up until that point) was completely gone without a trace.

The worry of it shutting down is annoying, but with the added threat of losing important work this is a crushing issue, and i'll really massively appreciate any help with the matter.

As for the system, as I say, I built it from scratch a couple weeks ago now, and haven't had any problems with it otherwise.
PSU: Antec Truepower 650w
GFX: Sapphire ATI Radeon 5850
CPU: AMD Phenom II x4 965
MoBo: ASUS Crosshair III
RAM: Pioneer Signature 8gb (2x 4gb)
HDD: Seagate Barracuda 1tb SATA
OS: Windows 7 Ultimate x64 (updated)

All components have latest drivers. I've had minimal software etc. running at the time of the incidents; above basic system processes the first was purely Mass Effect and Steam, and the second was Corel Painter 11 and Foobar2000 (lightweight media player), and Comodo Security in both instances.

I've run virus scans and such using both Comodo and Spybot and come up clean, which isn't surprising since I really haven't had chance to even get any viruses.

Also, no logs or anything are created on the crashes, so unfortunately I can't post anything like that.

So far overheating seems unlikely (just from what I can see), and i've double-checked my connections, so i'm wondering if it's perhaps faulty hardware? It'll be inconvenient if so, but I really need to fix this before it can do any real damage.

Thanks for reading this rather hefty post, and thanks loads in advance for any help; i'm desperate to fix this.

Happy christmas all, thanks again.

Tz
 
I would check your power supply with a digital multimeter. I have a Sears Craftsman that I go for $20 at KMart (great investment).
 
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