I know this is an old thread, and I know there are a number of threads describing fzj80s with similar symptoms, and many don't have resolution. Since these threads were so helpful for the 2-banana mechanic such as myself, I wanted to add to how I resolved the problem with my 93 that had similar symptoms. It took my months to finally chase down the problem. The erratic idle, engine hesitation, bumbling acceleration was not persistent, so it made it harder to diagnose. Plus, OBD code 71 (egr issues) poppep up from time to time.
I checked the intake tube for cracks, checked emmisions vacuum lines, used chevron techron fuel injector cleaner, replaced the engine coolant temperature sensor, checked the condition of the harness near the egr, replaced the fusible link, replaced the O2 sensors, replaced the ecu, replaced the air flow meter, and pretty much followed leads from many other threads related with this problem.
What I discovered was by chance. I pulled the dipstick out while the engine was running, and the engine immediately went into a stumble similar to what was happening erratically when I drove the vehicle. When I reinserted the dipstick, the engine ran smooth again. Then, I removed the oil filler cap, and the engine went into a stumble again.
So, it dawned on me that there could be a vacuum leak through any of these areas. I bought a new gasket for the oil filler cap, since the old one was brittle and could be allowing air in. That didn't fix my problem though. What fixed my problem was tightening the valve cover bolts that were loose. There were 2 bolts at the rear of the engine that were so loose that I could almost remove them with my bare fingers. Once I reset the ecu, and after a few drives, the engine ran like a champ. The 71 code never came back (it has been 3 months now).
I would add that replacing the valve cover gasket may be necessary if tightening the bolts don't provide an improved seal.
