I'd put my money on the receiver or the crystal. Yes, I know you said you get good range without the engine running, but it could very well be vibration causing the problem. The thing that points me in that direction is your saying that the glitches increase with more power. The only real change with more power is more vibration. Yes, the spark plug will fire more often, but I don't think the rx would notice much of a difference between 1500 times per minute and 6500 times per minute. Both are going to be multiple times per second, so vibration is most likely the culprit. Find a way to vibrate the receiver and see if the glitches come back. Sometimes even tapping it will cause the servos to jump. It could also be a loose connection that is aggravated by the vibration, but I do feel that the vibration is the source of the problem.

HTH
Jim