Eventually found problem with 1993 2.0 turbo 940 estate, which was suffering from occasional misfire between tickover to 2000 revs. Tried everything to sort, swapped out coil, leads, distributor cap and rotor, FPR, rpm sensor, cleaned throttle body, checked vacuum pipes were clear.

Luckily the misfire turned more permanent after a month of being intermittent.
Most importantly checked plugs after a run and all were clean and light brown. Plugs were 1 year old NGKs so didnt suspect them at all! However today after being convinced it was an ignition fault as it started and ticked over strongly on 3 cylinders and had ruled out everything else, I went through the leads one at a time, then dist cap - all short stop starts - then checked the plugs, of course without them being nice and hot the culprit showed up straight away - 3 light brown plugs and one really black one. I've had 25 motor bikes and 5 cars and never had a bad NGK plug let alone one that's only one year old! Just goes to show that sometimes you've got to expect the unexpected.

Thanks to david hunter who recommended checking the ignition elements based on the symptoms I described.

Only the air con, and cruise control to fix now!