Power Supply Troubles

Now this is a new one for me.
My computer loses power suddenly, but the monitors and everything else stay on. Puzzled, I checked my battery UPS and find it's got a full charge still. I turn my computer back on, only to have it lose power after a few seconds. I grew conserned, so I opened up the case and checked to make sure nothing was on fire, sparking, etc. Thankfully everything seemed fine, now I turn it on and watch as all the fans spin up, lights turn on, and the diagnostic on the motherboard completes with no problems. The computer seems fine, but as it's about to get to the log-in screen, the power supply fails again. I flip the switch on my power supply to turn it off and notice that it discharges electricity into the lights on the front of the computer (and presumably into other components as well). Then, after waiting a few minutes, I flip the switch back into the on position and without even having to touch the power button the computer starts booting up. I was puzzled, but the computer seemed to be running fine, a few minutes later the power supply gives out again.

I've got a definite problem. I half expected it from the cheap power supply I'm using, but I'd like to know if anyone else has had similar experiences or knows what is causing this. I'm about the go replace the power supply with a new one, but I'd at least like to know why a power supply that has been working fine for over a year decides to quit out so suddenly.

Just replace the PSU. Gone tru a few crappy ones myself.

One decided to spit out sparks as it's death knell. Few hours before that hard drives started freezing up and making weird noises, due to insufficient power.

Another was awesome and would randomly restart the PC at random times. Thought it was the mobo, sent it in for a week-long replacement >_>.

But anyway, replace your PSU and all should be dandy.