Well, from what you say it sure sounds like you have a radio issue. First check the obvious. As said above, check your battery voltage. Check all of your connections to ensure that you don't have a loose connection. Also, check for any metal to metal contacts that are vibrating in the plane or engine. Metal to metal vibrations can put out RF interference. If you can't find anything obvious as the issues then you need to GROUND THE PLANE until you have that receiver sent in for service. Issues like this usually result from a cracked complonent in the receiver, and the vibration of the engine causes it to fail. This is exactly why range checks should be done with the engine running.

Hope this helps

Ken