|OK, if it freezes in BIOS then you can at least forget software, which I guess is why you posted this on the hardware forum. |
There are all manner of possible reasons but one simple thing I would try first is to take out the RAM stick(s) and clean it(their) edge connectors with a soft pencil eraser. Pop them in and out a few times to clear any oxide on the sockets. Check all connections for firmness while you are in there, including those for the HD.
Always pop back and let us know the outcome - thanks