When I try to use the IC-705 on FT8 I get an error message saying that there is a problem with the sound card when I know the correct one is being selected. Probably there is RF in the shack.
My antenna is an end-fed, and RF in the shack is often a problem with end-feds. I have never had the issue before. It probably depends exactly where the leads are placed.
Tomorrow I shall try adding ferrite beads to the antenna and PC leads to see if this cures the problem.
One thing I should try is reducing the RF power as this should prove it. With far lower power it should avoid the issue of RF feedback. If this proves it, I know what to do to fix it.