I'll bet your throttle is going into failsafe due to the ignition swamping the RX. I had the same symptoms on 2 GS planes with DA 85's and spektrum equipment.

You probably bound the plane with the throttle position at low throttle, correct? In my case I was also blaming the throttle servo because I didn't realize I was briefly losing control (of the entire plane) but the most obvious symptom was the throttle servo would attempt to go back to idle (my failsafe position). All other channels should hold last known position.

I verified the issue by doing range checks on the ground with and without the engine running. Two different RXs and two different TX's were used in my case, which helped me narrow down the problem to my ignition noise. Initially, I swapped the throttle servo, but of course in my case it didn't help either.

As others have said, check your grounding on the spark plug. I suspect it's worn or you may have other IGN issues. You can verify my theory by doing range checks with and without the motor running (on the ground).

Joe