I had almost the same exact symptoms and can tell you my chain was stretched and worthless.
Also, please consider my earlier recommendation as I'm sure your mechanics did not check for a vacuum leak at the base of the intake manifold. Soak the intake down with carb cleaner where it connects to the head while running. It's an easy trick and if it hasn't been done yet, it needs to and will save you countless hours of scratching your head trying to figure out what the problem is. Let us know what you figure out.
1974 CJ5 all stock except for Rocky Road SOA
Completely for hunting - not for show or rock climbing