I wouldn't fly the heli at all until you've determined the issue with the radio system - this could be a disaster in the making. It sounds like this could be a possible receiver failure. A possibility is the circuit board in the receiver may have a stress crack (maybe due to a previous crash - or not). This may manifest itself as glitches when the engine is running due to the induced vibration. I would look to replace the receiver as an obvious "fix". Have you recently made changes to the radio components (besides the crystal you mentioned); i.e. orientation of the antennae, etc? Was this receiver ever involved in any crash?