A thought from my brother: Does your BIOS have any settings for waking the PC up from standby when prompted by hardware? This sort of setting is normally used for PCs used as fax machines for instance, so the fax modem can wake the PC up to receive a fax. Normally, these settings should all be off by default, but my bro had a mobo which had these turned on and, bizarrely, under certain circumstances the PC would power down, then back up. This was fixed by going into the BIOS and disabling all these settings. Since then, it has behaved perfectly.
On Gigabyte boards these settings are:
- PME Event Wake Up
- Power On by Ring
- Resume by Alarm
It's unlikely to be this, but worth a try?