Random Computer Power Off (Hardware related) - Help!

Newbie with just enough experience to be dangerous to computer needs help. Am running machine w/ nForce 680 sli Mobo, OCZ 700 watt PSU. Forget what type of memory i have. Worked beautifully for 1.5 years. Suddenly, I get back from Thanksgiving to find that the computer likes to randomly power-off itself.

Symptoms: None. No BSOD. No warning. Like someone pulled out the power plug (which didn't happen) or turn off the switch via power button. I can power it back on immediately and I might get 5 or 10 minutes of use before it powers off again. Sometimes while idle. Sometimes during intense use. And since I know folks will ask, have reset CPU to default settings, so overclocking shouldn't be issue.

Initially thought Vista problem (running 64 bit w/ SP1). But, it crashed on me as I was re-installing OS from Vista disk onto my reformatted C partition. So, I'm pretty sure it's a hardware issue.

What's the best way to tackle problem and diagnose the component(s) at fault? I'm 50% tempted to just build a new machine. But I really don't want to research all the new CPUs and mobos all over again just yet.

i thought of dust as a suspect, as well as maybe dust causing a short, so i sprayed off most of the major dust i could find. prior to me deleting off o/s, temp monitors for CPU and GPU were rock steady. there still may be dust causing a short somewhere, but it's not a temperature issue. assuming it's not dust, any other thoughts / recommendations? Is there a way to test PSU stability (over 10-20 minutes) w/o buying a new one? Any other potential culprits?

When is the last time you cleaned your system with compressed air? I mean thoroughly....

Beyond that, it may be your PSU or a half a dozen other possibilities

My first guess would be PSU. Could be that it is getting hot and shutting down. Thing may just have worn out. At least that's my initial thoughts. I'd probably start there.

clean the golden fingers of ram and video card, search the net for how to do it.
if persists,borrow someones PSU and test it in yours.
if problem remains, borrow a video card and use it.
if it still remains, borrow a mobo.

I say do power supply first, easiest. Then if thats not it, I say test your ram, one stick at a time. then mobo. I have had this problem with mobo before, but its the last thing you want to replace. I allso have had the problem with PSU