Installed Quadro 4000 (2 GB) and immediately realized why I upgraded to r9 390 in the first place.
At idle the temp was 70 degrees. Had to use MSI afterburner to push the fans to 100 speed for a temp of 50 degrees.
It uses 6 pin PCIe, so I cannot use Quadro and r9 390 together with my 700W PSU.
So Quadro goes back into the box.
Overall I believe it is a driver’s problem rather than GPU. Let me run R9 390 for a few days with newly installed drivers and check whether it reboots.
It is definitely not a CPU problem. It is GPU or drivers.