I'm having the exact same issue as hitmandn, btw.
Newer drivers give me the 4xx speeds, old blockchainers don't have any issue except for the quite obvious stalling.
Something very odd goes on once it needs to compile, like it loses the block.
I have another rig, vega cards that run through this issue no problem but also using the recommended drivers.
Odd, you are missing a step 100% because the rest of the world can run 470/480/570/580 just fine with 18.6.1. I have some test rigs on w10, 470 4/8g and 18.6.1 ..
Steps would be:
1.use ddu to delete drivers, and this is very important
2.install driver
3.patch drivers
4. set compute mode
Only these steps are required.
I always did its when install new driver, include driver blockchain, but it's still 4xx,. What different when used mod bios and original bios ??