Conductive grease (heatsink stuff) BAD idea it will get hot and leak down the side of the plug......guess where the spark goes then?
If the metal clip is loose then give it a little squeez. But first take a spare plug and see if maybe the clip is squooze to tight to go all the way down.
I know you don't to hear this now, but this is why we always recomend the shielded caps, no they won't fit it the cowl and yes they are ugly but they stop the RFi problems.
As an aside there are lots of BME and Brison twins running the rubber cap with no problem.
Different problem possible, can you install an FM(PPM) rx and do a range check?? Engine off max range and engine running range. This might tell us something.