Sounds like antenna bonding to me. Have you done a VSWR check of the antenna's from the T/R.
Might sound silly, but are the antenna attaching screws tight, and antenna is bonding to the airframe. I know some guys use the mounting screws only for the bonding. I was taught a clean ( no paint etc ) under the antanna base to airframe.
Dayton Granger make a two pack antenna sealant with aluminium balls to act as the conductive path for bonding. his stuff is the best i have worked with.