I had an '83 Southwind years ago which was prone to "stumbling" mine was caused by "cross firing" between cylinders. When the ignition wires aged, the spark could jump between a firing cylinder and one which was in the intake cycle, causing a sound like a "sneeze." New wires would usually fix it. I ended up buying NAPA lifetime wires because they would exchange them with no questions. I also found in a manual somewhere (don't recall where) that the routing of the ignition wires was important, keeping the wires between firing and intaking cylinders separated to minimize the cross firing.
One thing to realize is that the voltage required fireaplug goes up as the pressure increases, which is the case when the fireing cylinder is under full throttle. That makes the spark more likely to find another path. My "sneezing" always occurred under load condition.
Dick L
