Hi Phil and Welcome to the Forum.
Did I understand your last statement correctly - that it has the same issue running on LPG (propane) vs. gas? If so, then that rules out fuel filter or fuel pump. You don't say exacly which sensors (other than O2) you've replaced (MAT, MAP, TPS).
Have you seen the Check Engine Light come on while it is acting up? You should really try checking for trouble codes before throwing more money at it. Since your engine systems are pre-OBD-II you'll need a Ford specific code reader. This is a good one to get and not expensive either. You might find it locally for a few more dollars.
Amazon.com: Equus 3145 Ford Digital Code Reader: Automotive
Another possibility is a failing PIP sensor inside the distributor which is a pain to replace since the distributor must be torn down to get to it.
Good Luck and let us know what you find.