I finally found why the bike would occasionally run poorly. The screw that goes into the centre of the plug lead had come loose in the plastic plug cap. I found it because I was having trouble getting consistent continuity/resistance readings for the lead.
Munich Motorcycles here in Australia have copper core leads with 5K ohm NGK caps for AU$33 each. They are actually made for the BMW R65. This was damned convenient because a/ I own a R65 and b/ I needed some filters etc for it anyway.
The bike is again running very well.
