I had something very similar happen a few years ago.

See if you can find out if the metal gear in the muncie is still in good shape, if not, it will eat the plastic gear. And you need to make sure the screw on housing seats properly, if not, that will also chew up the gear as it won't be meshing properly.

If you hear a slight ticking when the car is moving, and it sounds like it might be coming from your speedo, the cable may be on it's way out. That too shortens the plastic gear life. If this is an original muncie (I had one in my '65 Chevelle 327 4 spd) you should suspect the drive gear first.