Check Engine light? Codes?
Exactly what spark plug brand & model # was in it before, and exactly what spark plug brand & model # did you just install?
I'm kinda leaning towards an 02 sensor problem but the wrong type of spark plug can cause misfires too... even if the guide book blesses them. Even Autolite has a bad recommendation in their spark plug guide for 2000 and newer TJ 4.0L engines so you can't always trust everything you read.