Dimension 8300 Powerup Problem

Having kind of a weird problem and not sure of the solution. Recently I all of a sudden had trouble turning on my computer (Dimension 8300 XP SP2). Had to hold the power button in for as much as twenty or thirty seconds before I could power up. Tried some different things and found that plugging into a different port on my USP seemed to fix the problem. Two days later the problem started up again. Took the computer off the UPS and plugged directly into a power strip. This seemed to fix the problem and then two days later the same problem reoccurred. I am at a loss as to what is going on. What is most confusing is how when I plug the computer into a different power outlet everything seems to be fine for a couple days and then the same powerup problems resume. I have not made any changes to my system, have run adaware and done a complete virus scan of my system. Any help would be greatly appreciated.

Jack

-- Jack Ruetty