I beleve the 9C is selectable modulation type for PCM as well as FM. If this is correct then you need to check that the wrong modulation type has not been inadvertantly selected. I am talking modulation type not modulation shift.

Check that the tx antenna is securly screwed down or if the type with the nutted stud on the bottom that the nut has not loosened.

The 9C is getting elderly and is normal to always rationalise the TX is never at fault but the cheaper and easier (especially now) Rx's are. Tx's really do wear out. Plastic cases dry up and easily crack for no reason, gimbles definately wear out and pots get dirty or corrode causing all manner of problems.

Before you start buying Rx's it would prevent a lot of frustration if you woud simply
try another RX only this time use a Futaba. Borrow one if you need. It would only take a few calls to me club mates and I would have one in hand for a test in short order.

I am sorry I certainly know its not what you want to hear but three Rx's all failing to function in short order its time to look at your Tx.

Perhaps its time to look at 2.4 there are many marvelous deals going all the time.

John