N4030 turns off

Hi,

I have Dell Inspirion N4030. From last couple of weeks, this laptop turns off after using for around 10 mins (in average). Sometimes it lasts 15 mins and some time 2-3 minutes only. The diagnositcs seems to be fine. I opened the laptop to see if the thermal cooling sink's fan works and it seems to be working fine.

Any idea? Where do I look?

Thanks

Paras.