You could hook up a small 12V bulb across your coil's primary and see
if the ignitor does its job (it is supposed to flash if ok). Once it
fails you will al least be able to determine if it is before or after
the coil.

Ignitors do fail from time to time -- not sure if they have an
intermittent failure mode because it looks like it is a darlington
transistor to me.
It is into the distributor. You'll see it as a module that has four
wires attached to it - it is mounted on a bracket that comes off with
two screws.

Maybe also check the coil, as the insulation can break down and it will
arc through the case to the distrubutor so perhaps remove it and
visually inspect it for cracks. I guess that could be an intermittent
failure mode.

I have a schematic -- it is out of a haynes manual. This one is fairly
accurate - let me know; I could email you a copy (schematic, that is)
if you need it.

Regards,
Remco