Inside the rubber boot that fits on your spark plug there is a metal cap the fits over the stem on the spark plug. Make sure that cap is a tight fit on the stem. Any slack there can produce a lot of RFI. If you have to squish it with the pliers but not so hard that you collapse it.
Also the stem has a button that screws on the stem. Make sure that is screwed down tight

Are you using a resistor spark plug? It should have an R in its number.
If the plug wire has a pigtail coming out of the plug wire next to the rubber cap, that should be clamped to the spark plug with a hose clamp