Sounds similar to the mining intensity issue with RX 6000 cards on windows 10. The Devs might have set the -mi value too high for your card. Try lowering the -mi value until you just start to lose Mh/s, for my RX 6800 it's about -mi 4.
What average hashrate do you end up with for your RX 6800? Have you tried the April driver + 5.6b?
64 MH/s on the dot, PM 5.6b and AMD 21.4.1. Mem set to 2150 and fast timings.