On my RB25 I have rich band from 1200 to 3000 rpm, when the air intake temp is below 5*, if I keep it above 3000 it's fine and it still idles well, only when the engine temp is below 60* that is, if I feather the throttle enough I can get to run smooth. It will burble upto 3g and clear, although I hate staying in that rev range with a cold motor its not for long as my tuner (Peter Woodward) tuned the microtech for cold start, but the temps were above these and so is doing a remap next thursday, I will let you know the result friday.
Does this sound similar to your problem or completely different?
It seems to me that it is an inherent problem with these rb motors even without the ecu.