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, GRT EIS, Garmin 340, 335, Dual 430s, FlightBox hard wired to WS, Dynon D10A with internal battery (backup EFIS)
Retired - Living the dream - And going broke!