OK, scenario is :-

MY94 STI, no significant mods except full decat exhaust (Mongoose). Approx 2 months ago changed from grade NGK6 to NGK7 spark plugs, just to try them out really. Once I'd got my local garage to get the gaps right (oops! :) ), I was getting a mis-fire in 3rd when using anything more than half-throttle, normally as full boost came in. Difficult to reproduce with any consistency as well.

Figured the coil packs were shot, so they got changed this morning. Runs much smoother, but now getting a similiar (but not quite so savage) mis-fire in 4th or 5th (ie. WOT in 5th from 80 to 90 usually triggers it). Still feels like one cylinder is failing to ignite. Between the the spark plug change and the coil pack change a new lambda sensor was also installed, in order to get through the MOT. Emissions as tested were fine, but the lambda sensor isn't the standard sensor - at £250+ I had to go for a cheaper option, although it isn't (I believe) the standard universal replacement.

The KL recently I installed "sometimes" lights up red when the misfire happens, but sometimes doesn't. Running BP SUL + NF booster, same mixture as the last 2 years so I'm certain the octane level isn't an issue.

So, the next plan may well be to try the "proper" lambda sensor, as the car does run rich higher up the rev range. However, what else should I consider ?

Check plugs : may have lost a tip, or cracked one when installed.
Fuel pump ? (Could be tired, and failing to hold pressure)
MAF ? (Not sure it'll be reading off this at higher revs though)

... Anything else I should be thinking about ? Not urgent, but a bit annoying.

I have / had a sort of similar problem after improving my breathing to the engine, was told it was a misfire caused by the extra air and I should put a smaller restriction in the hose off the bottom of the Boost Control Solenoid. ( < 1.5 mm )

Also told it could be the coils starting to fail ( they seem to age, may last 50,000 to 100,000 miles? ).

It is possible that the coil packs ( or one of them ) was not replaced quite right and is not making a good contact, or else was cracked during removal / refitting and is allowing a short, especially when wet?

Finally, ok, broken plug. Gap should be 0.7mm on 7B's.

Hope you sort it

Forgot to add, IIRC the lambda is not used after about 3000 rpm, the ECU then control boost from the MAF / MAP sensors for air and the map for fuel / ignition etc. Lambda seems to last around 60,000 from what I am told.

So bad lambda would affect idle, slow engine speeds.

Not sure on MAF failures, ECU seems to learn around them, gets worse after a reset. Usually fails towards lean mixture, hence engine threatening.