Dell Inspiron 1525 :: Sleep Mode Set When Lid Closed But Keeps Restarting

My wife and daughter both have Dell Inspiron 1525's.  My daughter's works flawlessly while my wife's has one irritating glitch. When the lid is closed, instead of going into sleep mode, it reboots itself.  It doesn't complete itself though it always stops at the "windows shutdown improperly how do you want to restart" or something like that screen...it's the dos screen that says do you want to start windows normally or in safe mode, etc.

When it does this it never shuts off so I'll get home from work or I'll get up in the morning to find the fan has run all night...It was purchased in august and I'd hate for my wife  to prematurely burn this thing up.

I've gone into ms config and one by one tried to isolate what the problem could be. Sometimes I have it narrowed down or actually got the culprit because once in awhile it'll do what it's supposed to do....then next time the lid gets closed it goes back to the existing problem.