I had the same thing a while back. The molex? connector (the power connector) was dodgy. As I pushed the connector in to mate up with the male pins, one of the female connectors was being pushed back. (Sounds sexual, I know.) Hence, I was getting a dodgy connection. As the computer got warmer, the coefficient of linear expansion clicked in and broke the power connection to the HD. Everything looked normal, fans running, etc. But the computer froze.
But it could be someting totally different.
Who says computers are mans best friend!!