Just got a 03 Golf

With the GPS the speedo is right up to about 65 MPH then it starts to go up and down by a few miles per hour, I cant hold the speed steady. I keep going faster trying to hold a steady - say 75 MPH.

When I try to set the cruise control it speeds up then slows down but it is constantlY building more MPH. I think the cruise is trying to hold a steady speed by reading the speedo but the speedo is bouncing up and down.

The cruise works good at 60 mph and the speedo is steady

Now for the questions

Does this car have a sensor for the speedo?

Where is it?

Approx cost?

Where can I get one?

OR

What do you guys think it is and how do I fix it?

Thank in advance
Birdman