my friend has a bronco and it gave us the same problem, it was the transmission. after changing the spark plugs, replacing the engine, checking the cat and all these other things that mechanics used tot ell us so we could spend money we found out that it was a transmission problem.
my boyfriend has a dodge ram 1500 and it does the same thing, sometimes in traffic the car would not be able to stay at a low RPM so it would just die. i say check your transmission, it might be it