I had a similar problem a Yaesu Mark V Field. It worked ok with low power, but not above around 20 Watts. I found out that it was stray RF. I ran my 600 ladder line differently from the tuner so that the RF was at a 90 degree angle and it works fine at 100 Watts now. I also grounded the rig to the tuner.
73 Richard W4MCD?
Richard W4MCD in NC