test the spark at the coil wire and see if one is markedly weaker than the others (i.e., #2). If one is not weaker, it's the cap or rotor or spark plug wire. You can have a bad cap and it will not show continuity to ground. The culprit is a carbon trace that leaks off the high tension juice but doesn't show on an ohmeter. Cracked rotor can also leak to the dizzy shaft and ground.

Not responsible for advice not taken...