K11 prefacelift idles ok, then all of a sudden low idle and smells like running rich

I know I have touched upon this before, but still can't get to the bottom of it with my stock Micra.
Sometimes it idles fine, then all of a random sudden the revs will drop and idle like it gonna cut out (sometimes it does), then smells like running ultra rich. If you hold your foot on the gas pedal sometimes it knocks it off and it goes back to running smoothly.

I have cleaned the throttle body (in situ), cleaned out the ICV, tested injector ohm resistance all ok, what would you say the next step diagnoses should be?