Been there too. Seems to be a design problem - happens often on small bore shafts with long spans. Shaft will "whip" with the smallest amount of fan unbalance.
If you can't have the fan (shaft, sheave and fan, all together) sent in for dynamic balancing you can try lock-tite (the red stuff) between the shaft and the bearing - then set the grub screws down like described above.
Done both repairs like this; both worked.