When I got my s12, If you booted it, bleah. it would just die.
if you drove it slowly though it went all right.
I had a talk with the previous owner and he said one day it just
stopped altogether, and he had it towed to the local sparky (not me!)
and had the ignition module replaced at great cost ($380!!!)
Anyway...when I bought it, I noticed a rubber hose at the back of the
engine bay had "fell off" that vacuum switch, so I put it back on.
Bad move. turned out the sparky had left the pipe off 'cause the new
ignition module wasn't the right one (a no-name black piece of s@*t)
and it only had provision for 1 coil.
every time the vacuum switch turned off, bleah. No spark.
If he had hooked up the coil that fires all the time and not the one
that turns off under load, nobody would of known!!!
As it was, I had a brand new bosch r31 pinny distributor, so I fitted that,
put the pipe back on, problem gone!!!