Check all your connections, make sure they are nice and tight. Also make sure there are no metal to metal contacts. When metal vibrates it can cause a RF problem. So if your using a metal clevis on the throttle with a metal control rod, change them to nyrod, attach an unused servo arm to the throttle(make sure its plastic) then you can use a metal clevis, the plastic acts as an insulator to keep the metal from vibrating and possibly causing RF interfernce. Also check you RX, is it at least 18 inches away from the ignition for the engine? The further away you can put it the better it will be. Also are you using a FM rx? If so try using a PCM rx, they are designed to block out RF problems and have a better recieving function. Just be sure you get one that will match your radio, meaning if your using a JR radio you have to use a JR PCM rx, same goes for futaba and hitec etc etc. Hope some of this helps.
MC
PS: I forgot to say make sur your TX is able to or has the PCM function to use a PCM rx.