So, recently my PSU has started failing. It was hard to diagnose, but here are the symptoms I got:

Randomly, but about once or twice a week, the system would hang/freeze and not respond to any input. This means that SysRq REISUB did NOT work, and the only thing I could do was hold the power button down to power down or press the reset button. There was nothing in any of the logs, and no warning before the hang.

These hangs could happen at any time, but were more common while playing video games (high system load). However, they also occurred while the system was idle, so it's not really a very good sign. During the hang the audio played in a loop continuously.

The only other symptom was while monitoring voltages. The +3.3V line varied quickly at short intervals between 3.30 and 3.32. I switched the bad PSU out with the one from my brother's computer and the voltage was STABLE at 3.33, while the PSU sometimes refused to boot on his computer. He had to press the power button twice sometimes to get it to boot.

I ran numerous other diagnostics tests, but all passed, so it may be a diagnosis by exclusion. I bought a new PSU and the voltage is stable at 3.35 V. The broken PSU was sent for analysis and either repair or recycling depending on costs.

I have added all this info to my hardware diagnostics wiki page:
http://docs.slackware.com/howtos:har...re_diagnostics

this is interesting
we have a computer that plays up
tech suggested power or psu

Last year I had a problem very similar to yours H_TeXMeX_H, took on average 3 starts to fully boot plus other assorted weirdness when it did. Checked what I could and finally bought a PSU from local Staples, Antec 450w (40-something dollars), to replace my 250w OEM unit. That fixed all problems, have since rebuilt with a new mobo/mem/cpu and it still works just fine.

Gives you an idea of where to start if you want one - PSU Calculator
http://images10.newegg.com/BizIntell...alc/index.html

I've had a number of failed power supplies. Some just die and need replacing. I've had a PSU die and fry the motherboard, cpu and graphics card all in one go, THAT was expensive :-(

I've had psu's do what you are seeing, basically the voltage rails become unstable, sometimes with a regular fluctuation and sometimes due to a current limit. This causes the voltage rail to dip when greater amounts of current are drawn, when the system is under load, that is often misinterpretted as an overclocking or overheating issue.

I always recommend buying the best power supply you can afford as the cheap ones are cheap for a reason.

I usually buy the best quality PSU available. I first calculate how many watts I need with something like:
http://www.extreme.outervision.com/p...ulatorlite.jsp
I add 100 - 150 W to that value and I buy that. This is also better in terms of efficiency. The one I bought now is a Corsair TX650 V2. It says on the box that it has maximum efficiency at 50% load and efficiency drops at lower and higher loads. I also may need the extra watts in case I change out parts or move to a new computer. You should also make sure your case stays cool, because it can only maintain efficiency at less that 50 C (at least for this PSU). The PSU is also quieter (lower fan speed) when it is not at 100% load.

Appears to be a good review site. They like to push the power all the way up and see if it "blows up" or not.
http://www.hardwaresecrets.com/page/power

Random & unpredictable problems are always signs of a failing power supply!

It could be RAM too, but I have run memtest and it passed.

Technically, now that I have a new PSU there are still hangs, but I can use Alt-SysRq to reboot the system, so maybe there is a software component, or maybe it is also the mobo that is the problem.

The old PSU was repaired and was confirmed to be broken before the fix...

Random and unpredictable problems can also simply be power supply, as in "the electric company."

I had a client for many years who operated out of a trailer. A photocopier was on the same circuit as the computer. The problem was solved in the short term by putting a high-quality UPS box on the circuit; then, by hiring a licensed electrician to re-wire the trailer with additional dedicated circuits for the key pieces of equipment.

I do have a USP, but I disconnected it because it was recently repaired after a failure. I wanted to exclude it as a factor. I suppose I could re-connect it, but other computers in the house don't have any issues.

Good point about the RAM!