The abs sensors are indeedy a pain in the rear, but just patience I'm afraid, and a good soaking in diesel fuel will help.
I assume your car is auto, the box uses more than just the speed reading, you may have a prob with the speedo itself as if it is registering speed can't see how it can be faulty at some speeds due to the way it works, a faulty turbine speed reading will also cause bad shifts, scorpios were horrendous for wiring probs related to boxes as well.
tabetha