Well........I think we have a winner!
Even tho this does not apply to our MJ's, I send it along for everyone's general fund of knowledge. After replacing everything we could imagine would affect the ignition, we have found that the alternator is intermittent. On the later model XJ's, the alternator is a part of the trigger for the coil. So, when it is not putting out, ignition is hit or miss.
Another lesson learned about haphazardly replacing parts. Oh well, now there are several spares......