Since you have a servo on your test stand it might be a good idea to use that to test for interference.
Take the receiver you are going to use in the plane to control the throttle servo.
Program "fail save" so that the engine is stopped when the receiver looses signal.
Then fist do a range check (some transmitters have a reduced power setting for that) without the engine running.
Mark the distance you get before you loose signal.
Then do it again while the engine is running.
If the range is less with the engine running its likely your ignition system causing interference.
Better test this on the ground outside the model before you risk your plane.