ORIGINAL: Flypaper 2
The cap/ braiding must be grounded to the engine or you will get interference. That's the ground for the module. A buddy took the ring off the cap where it's an interference fit on the base of the plug and got all kind of glitches. I soldered a wire to the braid and grounded it to the motor. No more interference. Just for a test, put an aligator clip jumper wire from the braid to the motor and see what you get.
If the plug cap is pushed all the way onto the plug, it HAS to make good electrical contact. This also completes the return path for the spark voltage to ground. The whole system is shielded and there should be very little RF energy escaping to cause radio interference. There should also be NO NEED for any additional ground connections. Just that simple.
Protect the shielding on the plug lead with spiral wrap or split lomb or something similar. But protect it at all cost from damage. If it rubs on anything it will wear through in short order causing damage to the plug lead.