please provide more information about the specific model of r9 390 and describe your systems airflow.
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Aftermarket cooling with temps of 80/90o?
Either the Aftermarket cooler is rubbish, you haven't installed it correctly, or you have damaged the chip. The 390 isnt a heat problem card, and I trust these temps are after some time at 100% load right? If so its a little hot but normal for a stock air cooler so go back and reinstall your aftermarket cooler with ample thermal paste on the naked GPU die (as opposed to a CPU with a cap needing very little in comparison, often overlooked). Most likely your issue here.
Polaris will be very light on power, because its a baby GPU.
Perfectly acceptable, anything 94c or lower is safe. If you want lower temperatures, increase the fan speed using Overdrive.