Originally Posted by Reznor
I'm not certain, but I was always told the standard copper plugs were best for these engines.
Whoever says that does not understand spark plug technology. They will often say that from having installed the wrong type of plugs & just go back to the stock plugs and then go on a mission to misinform the world that only standard copper plugs work properly.
The right platinum plugs perform well for 100k miles, the right iridium tipped plugs run well for upwards of 200k miles. Either will continue to fire in many conditions that would cause a conventional plug to misfire. And the right technology plugs depends on engine type, ignition type, and model year of engine. The right platinum plug for a 1997 4.0L engine, would not be the right platinum plug for the far more fussy 2000 and newer distributorless 4.0L engines.
CPRJeeper, that AP985 is the right plug and it works very (!) well in your model year and engine type. That misfiring you're experiencing can be from a broken plug, mis-gapped plug, bad ignition wire, miswired ignition wire, or maybe the distributor cap or rotor are not properly in place. I've been doing tunes in distributor equipped engines since the 60's and any of those can and do happen with even experienced mechanics. I'd just go over everything to find out what is causing the misfire... because it's not from the AP985 model plug you installed which is the correct model that performs very well.