Laptop Random Shutdowns during 3d GamePlay

well i have a dell d620 512 mb memory 5gig vm 1.83ghz and i can play diffrent game just fine but when it comes to WoW type games my laptop fan turns on really loud (like its trying hard to cool system) then laptop just turns off. the temp ranges between 100~134F

the ONLY time i can run these games is IF i lower my CPU speed from 1.8ghz down to 500mhz any higher and it will speed fan up then turn laptop off....any ideas whats causing it?

ps i have tried even setting a fan under laptop for extra cooling and it doesn't help.