I've seen LOTS and LOTS of bad electrolytic capacitors in power supplies as well as motherboards (they bulge on top and eventually vent. You can sometimes see this by shining a bright light into your power supply and looking around). Any power supply more than a year or two old is suspect. It's getting so bad that some companies, like ASUS for example, advertise their "Japanese solid capacitors" as a feature.
It seems the problem is due to really low quality capacitors from China. I'm guessing that they are not able to run at anywhere near their rated voltage for a long period of time. I've found them in lots of devices from DVD players, network switches, power supplies, motherboards, etc. This is a fairly recent problem (last 5 or 6 years). I've got lots of older stuff with perfectly good capacitors in them. I've even got some Motorola tube radios from the 50's with perfectly good electrolytic capacitors!
What happens the voltage drops slowly and the waveform of the supply voltage gets unstable. Then you start getting weird things happening. You could check the hardware monitor in your BIOS if you have one. If any of the voltages are more than 3 percent or so low then you should suspect the supply.