sounds like its time to use that warranty! does it do it even right after the ps3 is turned on after being off for a while? could be a heat issue i suppose.. is the fan running?? it could be something corrupt in the nand as well.. did you update recently? would be very risky if your ps3 keeps freezing.. but you could try finding the update pup and doing a system restore from a usb stick.
if it is a corrupt file or files on the nand that should help restore them.. but if the ps3 freezes during that process it will more than likely be screwed and HAVE to goto sony to get fixed.. but if it has to go anyways it might be worth a shot.. if it makes it through the update you could save yourself from being without your ps3 for a while..