It might also be worth checking the wiring itself - sometimes it slips and ends up resting against the exhaust manifold.
Still, when I've had that happen, I'd not call it "misfiring" - more like "cutting out entirely." It can be easily repaired with any convenient twin-lead (I actually used a section of 18/2 electrical cord last time!) and just tie it up out of the way of the exhaust manifold.
What year is your XJ? If it's 1991 and up, it should be OBD-compliant, and having a MIL can be instructive. If it's 1990-back, we'll have to troubleshoot...
5-90