If a hardware problem, and not a HDD issue, it could be anything, maybe RAM, maybe some transistor on the motherboard, even a faulty USB port.
If a software issue,
1) try a different version of win7. I used win7 Ultimate on many different configurations and found it to be very stable.
2) See if it works ok in Safe Mode (try working in Safe Mode with Networking for 4-5 hours). If yes, then certainly a software issue.