Bizarre shut down problem

Hi all, First post here - hoping someone can help me with what I think is one of the strangest problems I've had with a PC… been building them for over 10 years, so I've seen a few. I'm running Windows 7 x64, Intel Q6600, Asus P5N32-E SLI Plus MB, 4GB RAM, Nvidia GTX 275, Corsair HX 620 PSU. Problem started about 2-3 weeks back - I switched on my PC then went off somewhere, and when I came back it was still off, so switched it on again, and went through improper shut down message on boot and booted normally into windows and it was fine. But now it seems, whenever my computer has been off for a long period of time, ie overnight or when I come in from work, the first time I boot the PC, it boots normally into windows and after a couple mins of the PC being booted, it just shuts down, no message, nothing - more like a power loss. However, when I switch it back on immediately, it's fine, stays booted for hours without any other shut down. This happens over and over, first boot in a while it shuts down, and second boot works fine! Never seen anything like it really. I would have thought it was hardware, but hardware issues are normally more sporadic and this exhibits the same behaviour each time. Like I say, when it boots properly, I can leave it on for 24 hours with a problem. I did try running the PC outside of the surge protector and that made no difference. I have done registry clean, spyware clear and full virus sweep so everything is ok on that front. Ideas / suggestions appreciated!! Pete

only thing i can think of is to check the power supply and the capacitors on the motherboard.

PSU was my first thought. That motherboard uses solid polymer capacitors so "leaking" caps should not be a problem. Of course polymer caps can fail too, but there's no bulging or electrolyte leaking for telltale signs. If me, I would swap in a known good PSU.

Hmmm, Surely a dodgy PSU would show symptoms throughout the whole time that a PC is on? Saying that though, the only difference between the first boot and the second boot is the residual heat left in the components. It might be that once the components have "heated" up, it's fine. There may be a tiny short-circuit somewhere when the components are colder, and therefore smaller, than when they've heated up a few degrees. Not much more I could add to the advice that's already given.

Surely a dodgy PSU would show symptoms throughout the whole time that a PC is on?

No. Not at all. Power supplies can display, or be affected by intermittent events, just like any other electrical or mechanical device, especially when there are many variables involved (ambient temps and internal heat, varying loads, line-in anomalies, etc.). Whenever I see weird issues affecting hardware, I suspect the power supply. A power supply does not need to fail completely to take down a computer, or disrupt digital electronics. The PSU may be producing unstable, dirty, out-of-tolerance voltages that may be affecting a device expecting solid, clean, and within tolerances power. So when troubleshooting hardware, start at the wall. Is the wall outlet properly wired and grounded? Is the computer plugged in, and turned on? Is the PSU delivering clean power?

Thanks for the replies - will start with the PSU - it's my prime suspect also. Just need to find one to borrow to test with!

Yeah, if your supply lost a rail completely, then it would be easy to verify with a good tester. I keep one of these in my toolbag. This tester has an LCD display, which is better. At least then you can ensure your voltages are within the required tolerances. That said, none of these plug in testers are conclusive because they don't measure ripple, nor is the load realistic. But they are better than nothing.

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NOTE: Disregard the -5VDC reading. It is no longer used.