Various, random, and seemingly unassociated, problems on a computer would make me lean more toward the power supply. When the power supply starts to go, it can and will effect anything on the system.

I recently replaced the power supply on my daughter's Dell Dimension E-510. It did all sorts of funky things, and finally refused to boot. With a little help from my friends at Sysopt.com I diagnosed the trouble as the power supply. Her system is now running well.

IF you replace the power supply, buy only a good one. There are many companies out there making very marginal power supplies, and only a few making really good ones.

For computer help I go to my favorite computer forum, Sysopt.com. They've never steered me wrong.