My 98 xj starting having difficulty starting. It would stumble terrible and I had to flutter the gas peddle to keep it running. Then suddenly it would run fine but the Rpms would cruise up to 2000 then go down to 800. Then it would drive fine until the next time I tried to start it. The warmer the engine got the harder it was to start. It turned out to be the temperature sensor on the thermostat. It came slightly unplugged because the retaining clip broke. The 4.0 acted exactly like it had a vapor lock problem. I pushed the plug back on and restrained it with a zip tie. The XJ runs perfect again....... I hope this helps somebody. This situation drove me nuts for a couple of days...............Amazing how this simple situation created so much trouble.