Since you said that the problem only occurs during the receive mode then there is no problem in the antenna,cause if you have a problem in the antenna,you wont receive or transmitt and it will eventually burn your output transistor.So try checking your mic cable first.Since the cable has most of stress in use due to constant movement,so do a continuity check on the cable.Open the mic handle housing and do a point to point continuity check from the connections in the mic switch to the mic plug (of the mic cord).Next check the PTT switch.If the cable has an open connection,do some soldering work to reconnect the open wire.If the PTT is not working-replace with the same part.