This kind of soft fault is difficult to judge, and it is probably caused by aging inside the microphone cable or poor contact with the plug and socket. You can unplug the plug, clean it and plug it in again. You'd better change the microphone cable to see how it works. Otherwise, there is something wrong with the internal circuit of the power amplifier. For example, the connector of the microphone wire is in poor contact, or the performance of a component is poor or it is caused by welding.