I think you must have a bonding issue combined with some antenna shadowing.
Worse when the engine/prop cowls are between the transmitting station and your antennas.
When at right angles to the other station the antenna is probably not being shadowed as much by the fuselage, gear, prop etc allowing a bit more of the other stations audio through.Remember most bonding problems produce fairly weak signals but create localised havoc.
I have had several very tricky bonding problems particularly on VHF comms systems where HF ADF are unaffected.
If it only happens with engine revs it must have something to do with the engine/prop (or relative airspeed with revs at those levels).I also include what engine vibe may be doing to airframe bonding points.
I note no maint action was done to cause it and it is intermitant.This leads me further to the bonding conclusion.Maybe caused by eng vibe that only occurs above 2200.Just enough to break done a poor bonding wire or point.
Silly question, have you ground run the engine in the dark over 2200 to see if you have any flashover anywhere? Hard I know.
Does it only happen at altitude, reproducable on the ground, ever?
Please keep us informed.