Stop in for a cup of coffee

The starting circuit by-passes the resistor and switches over to power through it when you let off the key. When they begin to fail, resistance goes up and the spark from the coil gets weaker until they eventually are done. Testing them is unreliable since they can show good when cold and get worse as they heat up. Easiest thing to do is to keep a couple of spares and swap one out if you are having any ignition issue...like Tike says.