i shall just say this...very few understand spark plugs. they are a very passive device. succinctly a spark plug is a high voltage feed through providing a spark gap with a design-intended rate of thermal transfer.
when you have problems that you think are caused by the spark plug, they could be. so check them out with a VOM or a MEGGER. make sure the insulator doesn't spin in the shell.
but most of the time the cause of the problem is upstream. distributor cap. coil. ignition cables. spark plug boot.
have someone put a scope on the engine. it will tell you what you need to know.