since it seems to be after two hours it could be possible that it is a heat problem. Is the fan clean and still spinning? if you are worried about heat the first thing I would do is to upgrade the heat sink compound - use artic silver compound - way way way better than the blob of sh1t that microsoft give you.
ps you did not mention whether the xbox had been modded with a chip, bigger hard drive or anything like that. Might give us a bit more of a clue.