How old is this machine? Is the PSU properly sized (so it has reserve
capacity above what load is placed upon it)? Is is a good supply or a
cheapie? PSUs (good ones) lose about 5% capacity per year. The
cheapies are even worse plus they mislead regarding what they can
sustain (they give peak load capacity instead of sustained load
capacity). The PSU is the lifeblood of the computer. Too little
voltage (or too much ripple) or insufficient capacity to handle the load
(amperage) means flaky operation.