The description of the timing between when it hangs, and the fact it hangs faster after a hang and reboot would tell me it could be a cooling problem.

Can you take the side off and touch the CPU heatsink when it hangs? Be careful because if it is a cooling problem the heatsink can become hot enough to burn you.

If the heatsink it hot (like feels like it could burn you) then you most likely have a cooling problem and I would check all your fans and the seating of the heatsink.

Of course you could just check the temperatures in the BIOS if your motherboard has such a feature, but personally I like touching it

EDIT: doh, only just saw Tom has already suggested this. Anyway, give it a go