The misfire could be from:
- moisture in the spark plug well
- COP boot insulator
- carbon traces down the spark plug insulator
- bad spark plug including plug fouling.
- bad COP
- weak electrical connector
What many people don't realize is that IF there's spark escaping the boot insulator that if allowed to continue will CAUSE the boot insulator to fail. Once the insulation has broken down, then even if the original problem is resolved the misfire will continue.
I'm a big fan of starting an ignition tune up by just replacing the COP boot insulators. A full set of 8 is less $$ than a single new COP.
More Information for NGK 58901 $2.06 each (set of 8)
https://www.rockauto.com/en/moreinfo.php?pk=1596600&cc=1385636&jsn=439