Hello, I've experienced this same issue. Now, I have the
A1151 model (first generation of Macbook Pro) which does not have the NIVIDA video chip. While there are a lot of things that could cause this problem I take it that you have not opened up the top cover case. If you did then try these steps: remove only the two "allen type screws" completely (located behind the memory cover plate) also remove the four (4) screws located at the rear bottom side. Then start the machine and if it boots normally then just let it set idle for at least an hour. If all is well shut it down and wait for 30 minutes then reboot it again, let it just idle without logging in. If it stay up without shutting down then you know it is something pressing against the logic board underneath causing an automatic shutdown issue. If you have success, reinstall the 2 allen screws first then reinstall the back 4 screws only 2 at a time (the two closest ones on the same side). Then start the machine and repeat the idling process. Remember, this procedure is mainly for those who have gone inside of the machine to remove and reinstall the logic or I/O board for ANY reason. It will not hurt to still follow this procedure. If this clears up your issue, celebrate; you have saved yourself a boat load of money.