It might be overheating - do you often use it on a bed or on your lap? The fan constantly sucks in tiny fibres and may be clogged up inside. It would probably be best to take it to a pc repair shop and get them to have a look inside and clean it out for you (unless you are experienced/confident at taking it apart yourself). You can install a program called "speedfan"
http://www.almico.com/speedfan.php this will show you the temps your laptop is running at, normally it would be around 50/60 degrees. If it starts creeping up to 80s then shutting down this will confirm the overheating.