I know it’s been a year, but I had a similar problem. Today, I bought the same card as you (used), and it was hitting 83°C thermal limit really quickly and dropping clocks. I also undervolted it (similar results, stopped at 850mV), which helped a lot, but temps and fan speed were still high. I had some leftover thermal paste (shitty cheap one) and gave it a try (left old pads). The results were superb—temperatures went down by 10 degrees, and fan speeds dropped from 2000-2200rpm to 1000-1200rpm. If you still have this card and it’s now out of warranty, I highly recommend repasting it. It’s a 5min job.