This does sound very much like Radio Frequency Interference.
Do you still have a solid ident on the audio?
If yes, then the shielding on the harness between the KX125 and the KI209 is
in question. If no, then we have to think about antenna VSWR, etc.

I finaly disconected both KI-209A and KN-75 from my KX-125, but tracking a VOR
while transmit on COM still resulted "off" flag on KX-125's internal CDI.

I took a test flight to POC which has a VOR and ILS, and I found the following:

Transmitting on Com does not cut off ILS like it does with VOR;

When VOR went "off" while transmitting, there was no ident on Nav audio, the
Nav audio became completely silent; and

Pomona VOR is 110.40 and I transmitted on all fequency from 118 to 136 and each
fequency cuts off the VOR just the same.

Help!

Thanks,

Robin Hou