The problem is you only know when a coil pack has failed when you change it for a good one there are some tests that can be done but of limited value.
Fixed a similar problem on a 1992 MX5 the other day and it was the temperature sender that supplies the ECU with readings to decide when to put the electronic chocke on.
It was giving the wrong readings. In that case the simplistic diagnosetics gave 9 flashes on an LED.
On that car there was another sendor to feed the electric fan.
A friend uses one of these with his laptop on older Fords.
Yes change the plugs and if you can find a friend with a similar engine get a loan of their plug leads for twenty minutes. That would check the possible inexpensive route.
I pulled the spark plug cables and found water round the spark plugs. Probably from too enthusiastically washing.
After drying up I found the rough running still remained but to a lesser degree. A new inspection revealed lots of rust round the spark plugs closes to the bulkhead, could hardly see the hex head!
After scratching and vacuum cleaning round the spark plugs I also cleaned the spark plug cables which were covered in rust.
New test drive today, problem gone.
A cover over the camshaft cover and spark plug would probably be of great benefit.
I purchased a nice stainless steel cover that keeps most of the water out(and makes the engine bay sparkle), but I would suggest covering the engine when giving a good clean.
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