i have the same card as u.problem should be from something else.
i7 6700k/asus z170 pro gamer/sapphire r9 390 nitro with backplate
Scan for malware, bit coin mining bugs can cause the gpu to run at an accelerated rate. It happened to me with my 390x.
Malware bytes does a good job finding them.
Try with Trixx uninstalled(reset clocks to stock first). Hw Acc in browsers off.
What driver version.
I tried with the second version of Gpu's Bios, the first 5 minutes after power on of PC the gpu work "normally" in idle, using trixx 43° and 35% fan use(too much I think) with around 0% of cpu usage, approximately after 5 minutes the gpu % use increased and temperature increased without fan's increase and gpu went in safe mode.
I tried with another PC and no changed.
I followed your advice, using malwarebytes, install new driver removed the older using DDU(display driver uninstaller) but nothing changed.
I solved, during the operation of thermal pad replacement I saw that the Heatsink didn't touch the GPU, so I switched with pad of 0.5 mm thickness and was ok.
During the testing gpu usage was too high, I use malwarebytes and I have been hit by a bitcoin malware, I removed the malware and now it's all ok.