In the GigaByte BIOS, setting "PME Event Wake Up = Disabled" solved my constant Shut Down -> Reboot problem.
Now it shuts down reliably, but I don't think sleep is working quite right; the display and drives turn off, but the fans keep running. Seems like it's in S1 rather than S3 mode. But it comes back after pressing a key, so I can live with that. FWIW, when booting into Win7x64, sleep only works when Hybrid was disabled, otherwise, it sleeps for a moment then immediately wakes to the login screen.
I have a Belkin F8T016 BT transceiver, but unplugging that had no effect on the shutdown or sleep behavior as far as I can tell.