Do you have good batteries in the transmitter (if using the stock transmitter)? Just guessing here. Have you moved the receiver antenna? Maybe the antenna wire is broken? It sounds like radio glitches. Are the capacitors securely soldered to the main and tail motors? Make sure your crystal is seated completely in the receiver. Pull it out and reseat it. Make sure there are no wires rubbing on the main shaft and gear. Just some other things to check.

Chris