you are seeing a problem which is a result of the tx design -- you can not eliminate it .
I did "fix" it on mine by assigning /mixing both elevator halves as AUX (2&3) and nothing plugged into elevator.
The new DX7 does not do this .
the last version of the 10x still did it if you mixed EL with aux 4
most if not all PCM radios do it to some degree.
the least is the new Fytaba 14.
The technical reason is that the info for EL is fed at a different time than the later AUX channels and at a different rate in some cases.