GarageYears, I agree with your reasoning. I brought up the headset/intercom because it's not entirely clear to me here if we have a problem with an unusual radiator (RFI source) or a shielding problem (RFI sink). I do like your received signal strength thoughts.
What happens if we rewrite your 2) to be: Above 2200RPM something is generating an RF whitenoise signal? (intermittently)
Then we also introduce your concept of an antenna null towards the front of the aircraft (The OP does refer to the antenna installation location as "poorly positioned")
I wish we knew if its the the noise amplitude or the station amplitude that varies intermittently.