Speed Sensor?

I'm tryin to figure out what is wrong with my sister's car. I've been told a couple of times that it could be the speed sensor. For some reason when you start driving it is fine. Then every once and a while it will all of a sudden just sit there and rev when you hit the gas and it won't move. You have to put it in park, shut it off and when you start it, it might work. Does anyone have any ideas. Is it the speed sensor? I think it is a '92.

Doubt it is the speed sensor, sounds like the tranny is slipping. Is the tranny fluid full, clean, and not burnt?
The speed sensor tells the speedo and the computer how fast the car is moving and is used to tell the tranny when to shift, for the cruise control, and other functions.