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!

Seems like spark plug failure is happening more frequent now.

On my 74 144, had misfiring one morning 6 months after I rebuild the B20 with new plugs, wires, rotors and distributor. After checking, turns out one plug was not firing right, and was black. Replace all four plugs and no more problems.

Same thing on the 97 940. One plug failed less than 1 year of use. Car misfired on the freeway intermittently, but managed to drive it home. My very experienced mechanic pointed out the plug right away. Says he is seeing this more frequent as well.

All plugs are Bosch. Weird. Never had plugs failed on me in my 20 years of motoring until now. I make it a point to replace them at 30k miles intervals.

I used to run Motorcraft plugs on my 99 Ford Mustang GT, drove her really hard, and never had a plug failed prematurely.
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...and the bricks keep on rolling