98 Taurus Tranny problems

The speedo starts floating and the tranny downshifts. This only happens a couple of times a week but I want to try a figure out the problem. Talking to a friend of mine, he says it could be some type of sensor. Anyone heard of this type problem?
The transmission works fine when the speedo doesn`t start floating.

yes, this is an indication of a bad Vehicle Speed Sensor (VSS). This is a common problem around ~100k mi