How are you keeping your temps that low, my 390x sits at 93c 
Well, it depends a lot about where you live. My cards are around 55°C mining ZCash, 60°C mining Ethereum, and I have set the thing so that for 1°C my fans go at 1%, so 55% and 60%.
What GPU is that? I know its a 390X, but it must be aftermarket and not reference, shouldn't be running that fast.