Dimension E521 System reboots itself

Hello, I am working on a Dimension E521 System which has started to reboot itself for no apparent reason. The machine has been sitting unsed for awhile and now it reboots on it's own after being on for about 20 minutes or so. Sometimes during the reboot, it gets to the POST action then starts over again and again.

I've cleaned all the dust bunnies out of it and all teh fans are working properly. There is no obstruction of air to the machine. I'm hoping someone here could help before I spend money replacing parts...

Thanks in advance