i would say try the HDD, with reservations... a friend had a compaq which did the same thing. brought it into best buy tech support (cuz thats where he got it) twice and both times they replaced the HDD. so at least in some situations this makes sense to replace the HDD for random restarts...
of course they still havent fixed the problem, to my knowledge, as the HDD replacements did nothing but require him to back up all his data and reinstall everything... this is why i say to try the HDD with reservations... it didnt work for my friend so good chance it wouldnt work for you. if the HDD doesnt work... try booting with bare minimum components installed... and add in piece by piece until you find the conflicting piece...
if that doesnt work i don't know what to tell you...