Thanks, Cisco2, good advice for any PC. I have done most of those steps already. I dont keep a lot trash on my computer, just mainly use it for burning photo cds for my family. The reason why I suspected hardware as the root of the problem is that the reboots go away completely if usb devices are disconnected. Still could be loose connections.
I suspect that I will end up with a new case eventually. This acer just has no room for any other kind of powersupply. The hard drive is mounted just over the top of the ps.
I appreciate all who replied.