Ian and 1,
I had a very similar problem with my R34. Your description of it sounds exactly as I described mine. I went down the spitfire coil packs road etc etc to no avail. Do you have an after market air pod and exhaust system on the car?
I took mine to Andrew Coots at Pro-Automotive in Moonah, who was able to fix the problem. Something to do with too much air at the AFM, I would suggest you give him a call.