another computer problem

[cadpro]

Ok, I consider myself pretty computer savy, but this one has me miffed. Last week (on Thanksgiving of all days... ), my computer started acting psycho. Long-story-short, my computer (DELL) keeps shutting itself off, then restarting itself. Basically, you can use the computer for 15 minutes, and then it shuts itself down; once it does this, a few seconds go by, and then it turns itself back on, does its normal start up, and once it gets to the screen where you select the user, it shuts itself off again . It goes through this viscous cycle, each time, it going less and less in its start-up procedure. The only way I can prevent this is to unplug the computer from power.

Is this a fan/cooling issue on the CPU? or this is power supply (again, is it heat related?) issue? The fan on this computer is(was) so quite before, I can't say whether or not it is on.
Please help!