X2 or more on the plug cap note. Some use a hose clamp to really snug it down. Make sure you use the brand of plug the cap is designed for. Equivalent other brands may be just fine for the engine - but they have different shapes that may not fit the cap correctly. RCXcel ignition caps are designed for NGK only plugs, for example.

From there, it is a process of one-at-a-time elimination. Plug the battery in directly, and remove the kill switch. Remove the sensor. Try a completely different battery. Put the ignition battery way up close to the engine, and everything else as far away from it as possible. Eliminate all the conductive items (pushrods, wires, carbon fiber) that run from near the engine to near the RX, servos or their wiring as you can. Eliminate all switches. Eventually you will find the problem area, although it may be a long process of trying things and bench testing. Took me an entire summer for one setup to find out about the plug shape problem. Took three engine swaps (and many other items) to find it was a bad physical switch used for the ignition in another.
Check on-line in forums and directly with Spektrum about any reports of issues with the RX. Sometimes a firmware upgrade for some brands will fix a filtering problem, if one has been reported. At least your issue is pretty consistent - makes it much easier to troubleshoot than one that only shows up sporadically.