I've never had a transmitter go wrong on me, but unless it is defective to start out with, I would say things that CAN go wrong are dropping it (obviously), gettting it wet (flying in the rain or spilling something in it) or burning up the RF unit by leaving it on with the antenna down. I do that all the time in my shop even though the manufacturer specifically says not to and I haven't had any problems.

More often than not, servo jitter is a problem with the flight pack and usually some type of bad setup such as metal to metal that vibrates when the engine is running.

If you can't specifically pinpoint the problem then I would send the whole thing back and have them take a look at it. If the problem seems to disappear by itself then you can almost count on it returning when your plane is in the air which will most likely have a catastrophic outcome.

I hope you get it squared away. Nothing is worse than a problem that can't be found.