Did you check disconnecting the mic-cable or the rs-232-cable alternating- any influence on the problem? There could be two reasons:
1. some of the TX/RX are switching to transmit-mode when some pins of the rs-232-interface are going high or low, on some of them you can stop that function in the menu-settings.
2. unwanted PTT-activation is done via mic-cable, this could be a soft- or hardware-issue of the box.
Hope this will help!
73 de Mark, HB9AZT