I've come across this problem several times with Zens, both in cars and boats. People have come to me with curious radio glitch that no matter what they do to try to cure it it always returns, could be within seconds, could be in half an hour, but it never goes away for long. :-\
Do you know what the problem was each time? They were using a Hitec receiver. Swapped the rx for a Futaba one and no more radio problems at all. (Futaba and Hitec are both negative shift therefore the tx and rx are compatible with each other). Can't explain why it occurs, just know it does. Having said that though, I know of people who run Hitec receivers with them and have never had any problems.... But, to be on the safe side, I've always used a Futaba rx in my spark engined models and, touch wood, never suffered radio problems. I've no complaints about Hitec stuff, use it all the time in nitro cars and boats with no problems at all and 95% of the time always use a Hitec tx.
So, if you have a recurring radio problem with your Zen powered car or boat, I'll bet you're using a Hitec rx....
Kev.