1994 Ford F-100 serpentine belt

1994 ford f150 v8 5.0 the belt wont stay on the crank or engine pulley. It jumps one groove off towards the fan. So im on my 2nd belt in the past 2 months. I replaced the idler pulley, and water pump so far. All the other pulleys are fine but the power steering pulley "which is plastic" it has a few chips out of some of the grooves. Im pretty sure thats not the problem tho cause the belt goes around the water pump bell before it goes on the crank pulley. If u look at a diagram u'll see what I mean. So it rides fine on all the pulleys but the crank pulley.