Starting with the premise that it could be due to vibration:

Was the rx wrapped in foam (1/2" thick all the way around) or was the case actually touching the airframe?
Were the servos mounted on rubber grommets or directly to wood?
Is the prop fully balanced?
Is the engine solidly bolted to the mount?

Take care of those things and see if it makes a difference. Even if the problem was with the old rx, you still want to do the things in the list.