Years ago, had a slant six at work that gave me lots of grief. Turned out to be a clogged air breather on the valve cover that was forcing fumes into the intake manifold. Simple fix was a vented breather instead of a solid oil fill cap. Wasn't ignition that time, but points and timing is normally the first place I used to look. Malfunctioning fuel vapor canisters run a close second when I would inherit a problem child that others couldn't fix.