In similar case I just resolved, the R2 lead of the audio socket (PTT line) got dethatched from the pad on PCB. The CI-V configured units came from the same panel so it is possible that other units configured about the same time may have a weaker solder joint too. If you want to diagnose this scenario at home, you can remove the PCB from the enclosure and test PTT while pushing the PTT lead of the audio connector (closest to the edge of the PCB) against the pad.