According to Intel Processor Spec Finder:
a 2.8E Ghz Prescott CPU max operational temperature is 69.1C
here:
Linky
I would guess that at that temperature the CPU is already throttling to lower speeds/cycles. Max CPU load temperature must remain in the around the 60c to avoid throttling and overheat. Idle max temperatures should be around the 50C.
im guessing that thats probaby something to do with it, and in winter, inless the comps in a room with no heater on, its gonna be under thermal stress.
the power supply also is a valid point, and if your sure heats not the problem, grab a middle of the range power supply and see that makes a diffence - run something like 350/400 for some over head.. (real 300 watts will power any gaming rig(not massivly over clocked), 300watt is ~ what a stacked server needs)