Clean the heat sink of the video card with compressed air (like the aerosol can ones) if you haven't already. Dust buildup over time (especially if you have pets or if your PC is on carpet) can cause parts to overheat.

Check to ensure both fans on the RX480 are spinning, as it could be a cooling issue. If I recall that particular card model you have has easily replaceable fans if one happens to be malfunctioning (I believe the XFX hard swap fan kits on amazon are compatible).

If you are using fan curves, make sure that the fans are set to ramp up to at least 70-80% above 75C (if not set to 100% around those temps).

Case fan orientation can have a large effect on GPU temps, if possible have one of your case fans blowing air from the front of the case directed at the GPU to reduce the risk of the card recycling hot air. A positive pressure setup is also usually ideal (with those 3 fans, 2 intake, one exhaust. However, the ideal pressure setup can differ between cases).

If you are confident about electronics disassembly and reassembly: you could try removing the heat sink on the card and reapplying the thermal compound (would need to buy some, like Noctua's NT-H2 or Thermal Grizzly Kryonaut) on the GPU die. Depending on temps, usage, and time the compound can harden and be less efficient at transferring heat to the heat sinks.