I have that same problem with hitec 40 FM radios. We have allso tried Futaba radios with having the same problem. Once I tired my old AM radio and there was no problem. Now I have tried better spark plug and better ignition cable and no difference. The FG told us that they now this problem exist and their opinion is that the ignition causes the trouble. They are planning to make some shield over the motor. 1/5 scale cars seems to work because the motor is in different position. I really too hope that this problem should be solved as soon as posible!!