Continous Rebooting Problem

My computer keeps rebooting. It just keeps shutting off and restarting. I had one last year that done the same thing. Dell replaced everything in it and it done the same thing, finally they sent me a new one. It was just fine until a couple of days ago and now it's doing the same thing. Is dell having a lot of problems like this or am I doing something wrong? I've got a Dimension 8200 and have never had a minute problem out of it. The one that's rebooting is a Dell Studio XPS 435 MT. Can somebody please help?  It's sounds to me like it would be unusual to have two computers in a row to have this problem. Thank you