Rough engine operation could be due to a common part [ say, carb that feeds all 6 cyls ]....or....one or more cyl going out due to, say, worn rings causing plugs to oil up.
This is the BEST way to find the problem: a digital tach is very handy for this; many DVMs come with this as an attachment.
Simply hook up the tach to the coil lead. Engine idling, short out each spark plug one at a time. Note the rpm drop. If the rpm drop is about the same for each cyl, then you have a 'general' problem that affects all cyls. If one cyl has a rpm drop noticeably larger than the other cyls, then obviously that is the culprit. Problem could include head gasket, burned valve, wiped cam lobe etc, bad plug lead or spark plug, etc.
