OK, towards the end of the cruise Friday night car developed a significant missfire, started out being 1st 2nd and sometimes bottom of 3rd (all under load).
So saturday morning gave it a service (new oil, filter, plugs) and it was all good for a day. Next morning on the way back from maccas it started to miss in 1st under load. Still doing it this morning, it's nowhere near as pronounced as it was after the cruise and only does it in 1st gear if I mash the throttle hard.
Engine is RB30DET, splitfires, NGK plugs @ 0.8 gap (same plugs I always run), remapped ecu.
Next thing I'm going to try is a different igniter, mate has a spare at the workshop.
Anyone got any other ideas?