[nathanhu]

the problem is with these old coils and plug arrangement is that the contacts in the wiring plug (AMP type) start to corrode and build up a layer of oxydisation on the mating faces thus causing an earth fault to the front (mostly) cylinder.
the short term rectification is to disconect the coil and remove it .

inspect the inner contacts of the LT wiring plug looking for green tinge the the contact faces
do the same to the coil
spray a little WD40 into/onto the coil Lt plug
iff it happens all the time then twist the metal prongs inside the coil 1-2 deg to help induce a better / tighter fit to the LT plug