I own an APC Back-UPS ES 750; it's about 4 years old, and is the only major component remaining in a question that has been boggling me for the last 18 months.
(Yep, I originally posted this question about the possessed desktop, and while I marked a solution and closed the question, a week later the same problem returned)
I've tried plugging the desktop straight into the wall (but left the other components plugged into the UPS) and the desktop still froze. Is it possible that the EM field generated by the UPS is interfering with my desktop components and causing these otherwise-unpredictable system freezes?
To me this sounds like a long shot, but aside from my twin LCD monitors, that just about takes care of all the major components.