First post in here but I've read quite a few. Had 2 Skylines, 4 x C34 Stageas and just current M35 last 2 years. Missus still has a C34.
This one, VQ25DET, started missing about a year ago and a new set of plugs fixed it, completely.
Recently it has started missing when I get right up it - like passing with my foot down when the revs get over 5 grand.
That made me think about the fuel filter, if it was restricting flow when everything is sucking full thirsty.
Did another run that I do, bout 900 km round trip, and just a few km from home it suddenly started missing like a bastard on idle. Then it started missing under throttle and was ugly but didn't get any worse. I nursed it home in 3rd, spinning her over with light throttle to minimise the uneven-ness. Anyway, situation now is, it starts and runs fine - from cold - for about as long as it takes warm up the exhaust.
In about as long as it takes for the exhaust to cool off, it will start and run even again, but for less time.
Ran it for about 10 minutes as I parked it up on a ramp to get under it. After running longer, it would start and miss straight away 20 minutes later. Put my hand up its skirt - turbo was still nice and warm.
So it seems heat related and I don't know if it could be the oxygen sensor, a coil breaking down from running with huge plug gaps, the fuel filter - mmmm nahhh. Gave the air mass sensor a few whacks - that didn't fix it.
Any ideas or "try this to see if"s ?