Buck, although you do not say (correct me if I'm wrong) I assume you have the project working with some other '51. This indicates to me that your signal is hanging around some level (the threshold of the chips you cannot use) for some time. The cure for such a problem is not a one-shot (which usually end up shoting you in the foot) but a schmitt-trigger such as one sixth of a -14. If this does not take care of the problem, have some good looks with a scope at the transitions of yout hall-effect output and post again with info as to what it looks like.
Have fun
Erik