The internal resistors of Champions not only go bad more often than sparkplugs should, but Champion uses a unique “semiconductor” resistor that is hard to realistically test. I have had to use a 1000 volt megaohmeter, the old style hand crank megger.

If it is a Champion Double Platinum type, the little tiny discs crack their welds and disappear.

Sample quote

SAC-9 Suppressor
Champion developed the SAC-9 suppressor in the early 1980’s. This extremely reliable resistor/suppressor is formed from strontium carbonate, aluminum oxide and copper oxide powders. In fact, out of billions made, not a single SAC-9 suppressor has ever been found to fail in service. They are used in many Champion “R” type resistor plugs. An important feature of Champion plugs with the SAC-9 suppressor is that the resistance of the plug cannot be accurately measured with typical low voltage ohmmeters.

From

http://www.motospecs.com.au/assets/files/downloads/The_Champion_Difference.pdf

http://www.championpowersports.eu/assets/Uploads/II-Partnumbering-structure.pdf

Some of these newer style, long mileage plugs are designed to work with newer style ignitions, in that the juice needed to get a good spark might not be enough from a stock ecu or points. I'd consider the recommendations of whatever the manufacturer of your ignition system recommends when picking a plug.