How lean?
I know there was a lot of talk about coils but lean at idle points to an injector not injectoring.
To confirm what I already knew I literally just went outside, started my car up and watched the afr go from 14.0:1 up to 16.8:1 when I pulled an injector plug.
If the fuel kept going and the spark didn't then there would be excess fuel in the mixture and it would go rich. I cbf pulling my car apart to confirm but some things are just common sense.
Things I can think of doing off the top of my head right now (I've just had 3 beers so forgive me if I forget something or many things) are;
Swap injectors around to determine if it is a faulty injector,
Last I checked injectors have a constant 12V feed (give or take) and pulse the ground so check the the faulty cylinder has power when it's malfunctioning (multimeter),
Check that the ground is pulsing with the injector still plugged in (test/noid light or similar),
Check the pins in the plug aren't too wide, they can become bent and not provide a sound connection,
Check that the pins haven't been pulled out of the plug,
Those last two suggestions are tantamount to checking the wiring so check the wires for splits, cracks and breaks and also check the plugs in the loom for proper mating, corrosion and of course pin connection.
The only strange thing is that the problem disappeared briefly when you replaced the plugs.
Anyway as we all know there is only so much we can help over the internet and we are relying on your diagnosis and interpretation of the issue which may not be correct as well, no offense.
Good luck mate, hope this helps and if it is indeed a fuel issue, which it points to, then you have some ideas to keep you going.