Just bought 00 Cherokee sport 4x4 136000. Just had new cooling fan replaced, brakes, valve cover, transmission tune up. The truck runs great sometimes and sometimes I pull up at a stop light, it idles real funny, jumps up and down rpms and i take off and the truck wont run. It just idles foward, I give it gas and it does nothing then it eventuall after a 30 seconds of this sitting in the intersection barelly rolling, it'll move again. reving up and moving like normal. I was told it was the catylitic but the dealer showed me the paper work that it had been replaced so i just dont know. Please help somebody. Thanks

Is it the 4.0 liter 6-cylinder? I'd think Crankshaft Position Sensor. There are numerous threads on it, including a couple in the How To section (above the Cherokee section, when you first click on it)

Let us know if those descriptions sound like your issues or not....