[kaiser]

The resistance of plugs increase with age. It also increases with pressure!!

Many years ago, Champion (I think) made a small tester. It only needs compressed air and a power supply, and some sand. (My dear friend Hamish Morrison has one, he fixes RR and Bentley and Jaguars!! and has done for many years)

You can sandblast the plug on the one setting, and then fire the plug behind a glass. At the same time you can pressurise the chamber and watch the spark.

Often, on an older plug, it will fire happily at normal pressure, but fail, as the pressure is piled on.

So, my advice is to replace the plugs, it might just work!!