If it came with a plastic-cased, no-name ignition, try making a shield out of a magnetic type tin can. I once made a box out of a tin can(not an aluminum beer can) and formed it to the ignition. Then I soldered a wire on it and grounded it to the motor case. Problem went away. AND this was on 2.4Ghz. Some of those cheaper ignitions are just noisy. Especially on a smaller fuselage where space for separation can be limited.