1997 Ford Fiesta Misfire at low and medium revs

Engine is a multipoint fuel injection 16valve petrol 1.25 litre (zetec)
It misfires and runs rich at low revs - up to 3200rpm above which it suddenly runs cleanly. It is consistent (not changed by engine temp or weather conditions). I have changed air filter, the MAF sensor and checked operation of the Throttle Position Sensor (resistance increases steadily as you turn the pot). The compression is 185psi on all cylinders. I have looked for obvious airleaks but not found any. I have cleaned the O2 sensor. What is the likely cause(s)?