Help Hesitation/Misfire M3 E36 1994

I have hesitation/mis-firing at 2000rpm. Doesn't happen all the time, happens more if driven sensibly and can happen if the car is cold or hot. I have changed the 6 coil packs, plugs, o2 sensors, air flow meter, fuel pump, fuel filter, and have most recently taken the injectors out and had them cleaned and flow tested and all is well with them. Can anyone suggest anything as I have run out of ideas. If driven hard it doesn't appear to be so bad. Taken car to main dealer and they say there isn't anything wrong with it even though the technician test drove it with me and felt the hesitation himself!!!