PSU Shutdown issues: VX550W

About 6 months ago, I replaced a faulty power supply in my PC with a Corsair VX550W. I was very pleased, and everything worked fine.

About 1 month ago, my PC would randomly shutdown, and emit an obnoxious two-tone alarm. I eliminated issues such as temperature, voltage, and fan speed. Knowing that there were no recent HW or SW changes to my system, and that my VX550W PSU was plenty big for my simple system, I assumed that the problem was voltage regulation or flakey caps on the motherboard. I set off to purchase a new motherboard.

With the new motherboard installed, I would get intermitent shutdowns (still!?). Once again, I eliminated voltage, temperatures, fan speed. The shutdowns were hard shutdowns. The powersupply would just turn off. Going into the Windows event viewer, there was never any indication of an operating system or driver issues.

Knowing that drivers can sometimes cause strange problems, I downloaded and ran Memtest86+. It ran for about 15 minutes, with no errors, and the power supply shut down again.

The only thing left was the VX550W. In looking at this, the fan speed seemed fairly slow. When I removed the power supply from the case, the top of the PSU was very warm. I left the PSU on top of the case (instead of mounted inside), and ran for three days without a shutdown. I purchased a different brand power supply, installed it in the case, and have not had a single shutdown for 5 days.

FWIW, I've done some estimation on my power supply requirements, and I really don't think I'm pulling much more than 200-250W. I don't do anything very compute-intensive - mostly just web and e-mail.

In my opinion, the fan control inside the VX550 is not driving the fan fast enough, and I'm seeing thermal shutdowns from the PSU.

I would really like to use this PSU to build up another system.

Any other suggestions?