Problems with Ignition System
Worn spark plugs or spark plugs with an incorrect gap are a common issue, because the tip is delicate and spark plugs have a relatively short lifespan. If the spark plug gap is too big, the spark may not always be able to jump the gap between the center and side electrodes. No spark means no power on that stroke.
Ignition Coils (and distributors) convert the relatively low voltage of the battery into the high voltage needed by the spark plugs. These components may wear out eventually due to the heat generated each time they fire. If the insulation on the ignition coil is worn, all that electricity may wander down a different path, instead of going into the spark plug or wire as intended.
Spark plug wires are a common failure mode, as they wear out even faster than ignition coils. A worn spark plug wire may fire intermittently, weakly, or not at all.
Don’t discount the obvious, though. Make sure the spark plug wires are firmly attached to the ignition coils and the spark plugs. You may want to remove and inspect each wire to look for corrosion. Corrosion can place a gap between the coils, wires, and spark plugs, causing the engine to miss on that stroke.