It sounds crazy, but have you been messing with the power supply fans?
I used to have CTDs occasionally after about the same amount of time, and the only thing that had changed was that I'd moved house(!!!)
Turns out my power supply fan was blowing 'out' rather than sucking air in, thus removing smooth airflow from my case. My graphics card was simply overheating and after about an hour, it would conk out causing a CTD.
My new house (with double glazing!) happens to be warmer than my old one, thus prompting the overheating issue. Make sure your fans all pull in the same direction.
Otherwise, what they ^^^ all said...