RE: Transmitter changes from PPM to PCM by itself
I would return it to the service center and have them replace the PROM and batteries.
it might be your idleup switch or pitch curve that changes unrequested next time...
Someone at our field recently had such a case. Model 1 and 2 were OK. Model 3 showed broken but only once he was in the air.... The rest of the models were so damaged that he could not even load them anymore.