With a pentium II and an ATX power supply on occasion I do not get any power. Sometimes I need to only unplug the power cord from the power supply itself or change the cord and it will power on. Other times I need to unplug the power supply from themotherboard and plug it back in again. Then it will power up fine. Sometimes it will run for weeks, other times only days before I need to again unplug and replug. Why does this happen? Can it be prevented? Is it the power supply or the motherboard?
