speedometer intermittent

Having speedometer problems. When we start out the speedometer is fine, but after a short while it starts backing down. A lot of the times it goes all the way to zero, ticking up slightly. Other times it stops backtracking for a while, reading way below the actual speed, but not all the way to zero. Funny thing is, the Cruise control is fine. The Acadia has an digital readout on the dash when you "set" the cruise. That's how we figure out how fast we are actually going (that & a GPS). So... how do we fix the speedometer?

Thanks