You mention that you have everything grounded together. I undestand that. However, the technique you use is critical. All the receiver and servo circuitry must be grounded together at ONE point, and all the ignition circuitry must be grounded together at ANOTHER point, and then there must be one good copper wire between these two grounds. That is standard procedure in combined RF, analog and digital circuitry. Finally, engineers who work in this field only do so much calculating. Then they get busy and tinker. They try different things, because RF is a black art.