Having just had a similar problem with a gas engine and read what the OP has said it looks to me like the problem is he does not have a shielded HT lead or cap. I would suggest he replaces his ignition module with one that has a shielded lead and cap preferably one designed for an NGK plug. I replaced the cap on mine after having terrible problems getting the engine started and then to have the elevators go to full up when the engine did run, I fitted the new cap and soldered the braided sheath to the cap to make sure the earth path was complete. I found that tip on Just Engines site under replacing the 1/4 sparkplug caps. Hope this helps, by recently I mean first start Sunday replacement cap fitted Tuesday and flown yesterday.
Jim.
BTW I fly on 35mhz and after the cap change no interference at all.