Dell precision M6300 : random shutdowns

Hey I have used my dell laptop for like 5 years now. And it have all worked well. Until a cuppel of months ago. I was making a rendring with Cinema 4D When my pc suddenly shut itself down. So i er booted it agin and it worked all fine. Then a day later i was doing anothet rendring and suddenly my Pc shut itself down agin. I restarted it and hoped it was nothing. So When i then did a rendring, my pc shut itself down agin. Now i was being confused. So i tried to fix it myself, but it didnt work. I then after reading alot on the Internett, upgraded my BIOS. This only made it worse. Now Whenever i am trying to do stuf. It shuts down. I have cleand the fans for overheating but this didnt work either. I have tried every thing an nothing works. Pleas replie if u know how to fix this and if u have had the same problem Heres my system: Intel(R) Core(TM)2 Duo CPU T9300 @ 2.50 GHz 773 MHz 3,50 GB RAM Microsoft Windowes XP V2002 Hope you can help