Hi all... I have a p4 1gig ram machine with standard hardware (2 ide hd, dvd-rw, cd-rw, 128mb graphics and a few usb connections)

I built it in a soyo case with the stock 350w psu.

Everything works great, except I have one strange problem.
I can leave the machine on for days, rebooting when necessary without any problems at all. But sometimes, on a cold boot (rare occasion when machine has been off), it will start up, load windows, allow me to launch apps as normal, and then a few minutes later - zap - it restarts as if someone had pushed the power button.

This NEVER happens after the first few minutes, and NEVER on a reboot, only a cold boot.

I began suspecting something at the end of the startup routine, maybe something loading in the background that crashed the machine, but the other day it happened 5 times in a row, and the last 3 times never made it past the bios screen (so that rules out windows errors).

And then, it started up, and has been on for almost 36 hours no problems.

I should mention this machine is plugged into a small ups (that never fails during power outages) and everything is close to brand new.

Should I be suspecting the power supply? Seems like it would either work or not... but the cold reboot-only scenario has got me puzzled.
Could it be the ups?

Any light anyone can shed on this would be greatly appreciated, I dont want to start taking things apart without knowing the most likely suspect.

Thanks very much... Michael

Give us the details of your powersupply. If it came with the case, it is most likely one of those throwaway doorstops. They work reasonably well with a minimal setup, but only just. Make any demands on it, and it's bye-bye.

could it be the ups?

decided to experiment by unplugging the pc from the ups and going straight to the wall. cold booted fine, 4 times in a row.

before I go any further I will try this for a while and see if in fact it was the ups itself. If the problem persists, I will certainly be back.

thanks for the input so far.