Same problem, here. I have been through the wringer with this old gal (93 Cherokee 4.0 with 250,000 miles).
I've replaced the Crank Sensor, the Cam Sensor, Timing chain, coil, wires/plugs/rotor & cap and the ECM (computer)! Still a problem. I suspect a wiring problem as I have already had to fix the junction where the injectors tie together with the lead from the ECM. Took me two months of afternoon and weekend probing with a VOM to finally find it!
Anyway, if anyone has a quick fix or a sure fire diagnostic approach, I'm all ears!