Can anyone tell me why my engine would just die driving down the road at about 55mph. (Just sometimes). Or what would cause the jeep to have a jerking motion at about 55? It only jerks once but then keeps driving. It's like an abrupt slowing down motion, like it stalls and the auto tranny seems to grab or somthing..and sometimes it even dies!! It all happens from 45 to55.

HELP!!
93 Cherokee sport 2wd
4.0L H/o I6
a list of all I've done to it
New oil pump,alternator,waterpump,tires,
used tranny and torque converter,new aux fan
spark plugs,air filter,new fuse panel and wires
new fuel pressure regulator

does anyone else have this problem?