The life of an ignition coil...

I'm curious to know what causes an ignition coil (and HT lead) to give up on life.

Riding along the A13 last night, 20 miles into my journey and the bike dropped on to the LH cylinder only. The RH one kicked in a mile or so later but soon ducked out of the party again and I completed the remaining 3 miles on one cylinder.

A few hours later it was time to return home, bike started fine and idled without a problem. 15 miles into the return leg and back on to one cylinder (I assume running on the LH one again). Occasionally the other cylinder would put in an appearance but never for longer than a couple of miles.

I'm used to TDMs not behaving in the wet but my problem was happening in the dry. Also, the wet weather misfire caused the bike to stall whereas last night my bike was idling (even if only on one cylinder).

I'm just about to go and poke around, I'm guessing a loose connection or a duff coil as the most likely culprits but would appreciate any thoughts you lot might have. I guess my curiosity is because I'd have thought a coil would either work or not, no intermittent crap.

FWIW, I assume the coils are the original items and are 40k miles/14 years old.

A number of things could happen. The wire inside the coil is fine and made of copper, which doesn't take well to vibration. It is supposed to be potted in place but the effects of heat-cool and chassis vibration could cause a wire to crack. The current then jumps the gap, progressively eroding it until it arcs and burns out. Or a crack in the potting could allow moisture in, again leading to internal arcing and loss of continuity.

Failing at temperature cold be expansion of the coil assy causing a gap to develop on the LT side (low voltages don't jump gaps well).

You can test the resistance over it with a multimeter to rule it out. I've had one go before, the airbox had a little drain hole in, right above a stick coil, rotted/rusted solid.

Cheer up, my Mondeo went though a coil pack per year the whole time I had it, I kept the reciepts and got a free one each time.