I also have sony pcg grz 630 and underwent the same trauma of overheating...was sick of it,. after playing need for speed game for only 2 minutes, the computer wud just shut itself down...Solution???
go to staples/officemax and buy a duster (compressed air can) for the laptop ,comes cheap
and get all the dust out of it, dont be afraid to empty all the can , get dust out of everywhere.
My computer now works excellent, just played NFS 7 for continuous 2 hours & the computer is still not even warm enough..so..enjoy cleaning !!!