Your drive face is slipping while the fan is spinning with the crank. Try hitting the end of the boss, and the area of the face that contacts the boss with a piece of 60 grit sand paper. Just enough to create rough surfaces to "grab". A dab of green lok Tite on the mating surface may help too.
Crazy that they spline the fan, and NOT the drive face.
Try the ZX face. It is splined. No need for the fan then either.