This is the exact reason I still suggest to seperate ignition components and RX components. Everything is fine until ONE item starts acting up and then RF gets into the RX. The ignition or the optical switch has an issue and is flooding the RX with RF noise. Removing the kill switch will most likely fix the problem. Do check the plug cap for damage and the braided sheilding on the plug wire.

Sounds like you are using A JR radio as they are the only brand that I am aware of that suggest a range check with the antenna off. Continue to range check as per JR instructions.

I know alot of people will not fly a gasser without an optical kill switch, I never have. I usually set the failsafe to close the carb and then run a choke servo. IMO a link between the ignition and RX is established when using one and for me I just won't go there. If you must run one take a look at some other brands. Smart Fly would be at the top of my list due to the fact that if you do have an issue you can call and speak with the owner/designer directly.