The PSU is under very little load just sitting there running memtest.

But with SageTV up and running, recording a program or two, and transcoding or commflagging them, things start to suck a little more power.

I'm betting that eventually something gets hotter than it wants to get (a failing component or whatever), and then it misbehaves until it gets to cool down substantially again.

PSU is the most likely candidate, but it's hard to tell without being able to swap in a known good PSU.

Cheers