A big drop in RPM when the ignitor is removed usually indicates that the idle mixture is set rich. IF the F7 is working fine though I would just stick with it and fly. Your A3 plug may have gone bad or the engine design may just work better with the F7 (I am not familiar with it but it sounds like the F7 may be a "hotter" plug).

FYI, other symptoms of a rich idle setting are if the engine wants to load up at idle, and/or stumble as you increase the power from idle. If you are seeing this even with the F7 then you definitely should try leaning the idle mixture a bit.

Walt