Your post suggests the PSU is the culprit. Considering that the 939 is roughly five years outdated, I'd be willing to bet that your PSU is just as old. If the fan at the back of the PSU is caked with dust, you should clean it.
If the CMOS battery is old, it can also cause the problems you are facing. The average life of these batteries is three years. Try replacing the battery. You can pick one up just about anywhere batteries are sold.