If it is a bad plug, that is an easy fix. Try that first.

You can test the coils by doing your same method... swap to another cylinder.

I assume you are getting a CEL after you misfire... what do your codes say?

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2002 X5 4.6is - Titanium Silver / Black Alcantara - Dinan Stage 2 - Bilstein/Eibach - Brembo BBK ----SOLD---
1993 318i - Brilliantrot / Tan - bone stock daily driver