Sounds like it's overheating, does it feel really hot? Is there any warm air coming out of the vent on the back/side of the system? I'd guess that either the fan isn't working, or the heatsink is plugged with dust. I'd say that dust is the most likely cause, however, as this is a very common problem.
The dust problem can be prevented with canned air on a routing basis, however, once it reaches the state of being plugged, the only solution is to take apart the laptop and clean it out manually.
If it's still under warranty, go that route, won't cost you anything. If not, unless you are experienced with laptop dis-assembly, you will probably have to bring it in to a capable computer shop.
If you feel you are mechanically inclinded enough, you can find a service manual with step-by-step instructions on dis-assembly under Manuals in the support section on Dell's website.
Just enter your service tag, or search for your model at the link below...
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