For the reboot problem make sure to reset BIOS. If that does not help try to unplug all USB devices apart from mouse and keyboard. If that does not work check the "Wake on LAN " option in BIOS and turn it off.
As for the card temp it is normal because R9 390 has a 0db mode which turns off the fans and only turn them ON after certain temperature is reached so there is nothing to worry about.
mh, didn't help but thank you anways!