Radio issue
I assume you have resolved this by now, but here are some thoughts. The tower can 'hear' your carrier wave transmission (or broken audio), so the problem appears to be a loss of MIC audio. The signal for the audio I suspect travels from the MIC in your headset, though the headset connection, to the intercom and from there to the appropriate radio for transmission (the Narco in this case), from the IC it arrives at the Narco and inside that unit the MIC audio is added to the carrier wave and heads off into the ether. Somewhere along that path you have a break.
To start - If you can hear your own voice without any breaking up etc. over the intercom you can assume the headset and the headset sockets are good (they are common to the radio transmission of course), that means you need to look more at the Narco or the connection to it from the I/C.
Another quick check, you did not mention whether you have a P2 position, can you plug yourself into that and transmit from that side when your experiencing the problem? - That MIC audio will be a different input to the I/C and may work better if that P1 channel of the I/C is intermittent in operation.
Once again not sure of the unit itself but if you have an emergency position on the I/C then try that when the transmission issue is occurring, that will essentially pass your audio through with the most basic of addition from the I/C and if that improves matters then your I/C must be suspected.
After that lot your down to the Narco and there is not a lot you can do with that other than borrow someone else s for a flight if that possible. With the Narco extraction PIN they are very easy to remove and install.
All the best
The Doc