Help Please==Taurus Speed sensor problem?

I have a ford taurus,which gives jerks(Gear slips) when the engine warms up a little bit. It just slips the gear. And recently i started to get the service engine light. I took it to the mechanic and he told me that the speed sensor is bad. He is not sure if that would fix the problem of the loosing power.
I am not sure if he is telling the truth? Do you think that replacing the speed sensor will actually solve the problem of the gear actually slipping? Do you think that both are linked together?

THANKS.

Replacing the Variable Speed Sensor is a fairly inexpensive way to start with this problem. First there is a really good chance it actually will fix the problem and the part is only about $30. Two things. One: When the VSS went out in my wife's Taurus the speedometer stop working; from 55 to 0, it was the wildest thing to watch. I got to see it the second time the VSS went out. Two: The VSS went out the second time because I wasn't careful about replacing the heat shield that protects the VSS from the excessive heat generated by the engine.

This part is not much fun to replace. You have to work almost entirely by feel while leaning over the driver side front quarter to reach down by the firewall side exhaust port. You'll want to do this job when the car is cold.

Hope this helps.