I just fixed without solving the cause of a high rpm misfire that’s plagued me for months. It kind of coincided with my MKZ turbo swap last winter. Perceptibly poorer running I attributed to winter blend fuel as previously experienced. Then I lost my job to COVID in April so my driving miles went to nearly zero. Then the racing season was delayed until June. Had great improvements year-over-year after the turbo swap but had a nagging misfire issue just like yours that also was aggravated by a transmission shifting issue (2-3 shift slurring or delaying). I’m on Livernois tune, 93 gasoline only. After tracking down the transmission problem to a leak resulting in low fluid levels, I replaced original coils and stock plugs with MSD and NGK. Then switched back to used, but cleaned and re-gapped stock plugs, then pulled all coils and plugs to methodically move them around and persistently got misfire on 3 & 6 and general misfire codes. Eventually CEL on continuously. Finally had to return to Ford Stock so the dealership could do in car monitoring. My mechanic drove 69 miles of monitoring, checked every single thing that could be checked. Not a thing out of whack but the spark. Then he put in a brand new set of stock Ford plugs, gapped to factory spec, plugged in the MSD coils, and poof no misfire. That was last Monday & Tuesday. I have not reloaded the car to Livernois tune yet. But Ford stock with all my other mods it still runs very strong. No way to test it with my local track closed. But no misfire for the first time since August. Clearly it did not like the NGKs. And evidently the old Ford plugs with 35,000 miles on them weren’t cutting it either. My mechanic and service manager are completely involved with my build and neither of them had any clue what was actually causing the misfire. It seems like my car eats plugs. This is the third set of Ford plugs, excluding the NGKs that were in for 7500 miles.