Nissan Micra technical advice request on; - k11 micra misfire, at 30 mph only

Hi all,

This 1996, 1000cc. with 68K miles only, consistently misfires between 27 to 33 MPH under various load conditions. Backing off the throttle can minimise the misfire but not eliminate it. Apart from this, the vehicle starts & drives normally. Does not burn any oil or lose any water, both are uncontaminated.

Remedies tried without success are:

Checked wiring, plug connectors, fuses/holders, & all engine bay earth lug points.
Cleaned TB. MAF. hot wire & air temp sensor with IPA.
Re-soldered TB. 6 terminals for hot wire / temp.
New plugs / HT lead set & new petrol filter.
Cleaned out distributer, rotor & cap segments.
Fitted s/h replacement TB. & Distributor.
Set timing @ 15 deg. Advance with strobe, as per Haynes manual.
Charging alternator volts steady at 13.8 V maximum.
Never had a check engine light.

Noticed that the timing advance is unstable in that it snatches rapidly towards maximum when the RPM is gradually increased from steady idle tick over to 1500 RPM approx ?
Timing is not erratic on throttle snap opening & snap closing, not scattered. Chain rattles a little but does appear to scatter the timing under the strobe.
Exhaust tail pipe is clean & shiny indicating a rather lean running mixture for an old car. ECU.,controls timing, but cannot see this at fault as vehicle performs normally in all other instances.
Now out of ideas. Need a more logical test & evidence gathering procedure going forward.
Any proactive ideas please ?
Micra Paul