I just remembered that those Cherokees have a one piece coil over plug coil pack. I remember back when I was working for dodge, we had to replace a bunch of those because they would cause a random misfire. Just a thought, but I'd check the voltage and signals going to the coil pack. if the signal isn't present while cranking the crank or cam sensor could be causing a problem, but if they are getting correct signal, than the coil pack is failing. We always recommended replacing the coil packs on these things as a tune up item. about 100,000, then replace. Just another thing to check, hope it helps.