Motor is in a 166 Super, on a T-Plate, with about 95k on it.
When I picked up the car it had what I thought was a misfire at around 4250RPM.
It has had fresh oil and filters.
It has had fresh OEM-spec platinum plugs. The old plugs did not look well.
The split intake pipe between the airflow meter and throttle butterfly has been repaired.
It has been hooked up to diagnostics and live readings were taken, as well as fault codes reset. All seemed well on the diagnostics once the codes were cleared - these were the codes:
The ECU has been reset, using the push-turn-dangle-yank procedure outlined on here.
A new/secondhand coilpack has been sourced and rotated through the six positions to see if it was a coilpack taking a dump under load - not much different throughout the rotation.
It is a bit grumpy and a *little* miss-y when cold, after being left for a couple of days, but after the first half mile everything is happy, until you poke it past 4250RPM, when most of the time it'll flat-spot/miss to varying degrees, sometimes I change up, sometimes it pulls through it, sometimes it gets a bit worse.
Any suggestions on what to do next would be gratefully received. It feels like there's a great (big) car in there, trying to get out. If I can cure this issue, I think it'll come out to play.