I'm not sure this will help, but I had a '79 Jeep that did the same thing and I tried the same things you have done. I finally replaced the gears and timing chain and that fixed it. It may be just that your belt is old and need replacement. Hope that helps.
Check distributer grounding wire connection, check reluctor
pickup sensor coil check cap carbon tracking check firing
order if some one worked on engine recently. Check HV coil
Clean plug wires test replace if high leakage observed.
check engine timing. Check distributer mech slop or wear
This sounds like the same thing my 88 did when the little vacuum hose that goes between the intake horn and the intake manifold fell off. Similar symptoms have been described elsewhere in this board. If this is your problem, it's an easy fix!