00 Cherokee Sport 4x4 dies out but stays running

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....