This raises some flags. It's a poor PSU. One of the "best to avoid" PSUs. It's best if you upgrade it to a better one regardless if it's what is causing the poor performance.
How are your temps while gaming and in idle?
Use DDU in safe mode to remove and reinstall your GPU drivers.
At least from what I've usually seen and read, Gigabyte RX 580s seem to tend to have inferior heat sinks at times. IIRC, the reviews on them were normally more non-favorable than not. Was the 70 degree part idle or while under load?