Double check all of your power and ground wires. My 360 powered CJ had the same issue, turned out to be a poorly crimped lug on the end of the ground wire. Where's your battery? Still under the hood? The main ground should be direct to the block, then ground wires from the block to frame and battery to body.

Carry a set of jumper cables. Next time it happens, run one of the leads directly from the battery negative to the starter body, and see if it cranks normally. That will prove a bad ground elsewhere.

Bob
tufcj