I have delineated a number of things I don't think it can be, lets see take a shot at what I think it can be.
If you have tried another servo and it still does it, it might be a loose connection in the output receptacle of that receiver channel. Or an intermittent connection in the electronics for that channel. Plug it into another channel and see if does the same.
If you still have the problem, then the only thing it can be is the ignition unit is putting out so much garbage it interferes with the pulse that is coming from the receiver. If the shield on the plug lead is intact and the cap is firmly on the spark plug, get rid if the ignition unit. Its an accident ready to happen.
Hobbyking has ignition units for about $25. I have three of them. They work fine.
Lets see the PRF of the servo is about 20ms. Thats about 3000rpm or high idle speed. Now if you increase the motor speed to 6000 the servo would get twice the pulses if the ignition unit is putting out that much garbage.
Sound interesting?