Constant reboot

Sorry, I have no definitive answer. However, I can confirm your symptoms.

I also started having restarts occur after choosing [Shut Down]. It was also a Gigabyte board, 88GMA and it was Ubuntu 14.04 at the time. Oddly enough, I noticed a patte that it happened regularly from the panel button but I never saw it happen once from the Cairo Dock power icon. Could have been coincidence or it could have been a clue beyond my ability to process.

I can tell you that it went away after a complete fresh install of 15.10 (done for other reasons) but that is an extreme fix for your case.