I had a similar problem (I think) a while back. What fixed my problem was the connection from the lower comm antenna to the antenna itself was frade and once I replaced that connection with a new connector the problem went away.
Jim Harris, RV7A, 2nd owner, N523RM
IO-360, Hartzel CS prop, Older Aerotronics built panel Dual GRT Horizon WS, Garmin 340, 335, Dual 430s, Stratux hard wired to WS, Dynon D10A (backup EFIS)
Retired - Living the dream - And going broke!