For what its worth, I found out that 80MHz on 5GHz is buggy and if you don't need 866Mbps+ speed then switch to 40MHz and setting channel to 44 is working great for me. I also have 0 problems on 2GHz channel set to 1 at 20MHz. Both radio power levels are maxed out.
I'm on snapshot build with default kmod and QCA packages (-ct pre-installed) on Archer C7 v5.