- the machine doesn't warn me about overheated processors or anything when it reboots (I mention this because when my CPU fan was breaking down it would freeze or reset the system and then it would give me a temperature error on startup, and I know it isn't that this time)
1] So have you replaced the fan since? And if u have has this restart problem started since?

2] Heatsinks collect dust and sometimes that could be it.

3] Check whether all the fans in the case, on the vga card, cpu, etc, are working.