Pictured generic image from the inter webs

My speedometer is giving me issues. Sometimes when I drive down the road the MPH is 10/15mph too slow on my gauge clauster. Other times it is fine, and occasionally it is more like 5mph off. This appears to only be an issue with my speedometer the rpm gauge is fine (when gps says I am going 55mph the gauge correctly reads roughly 1700pm even if my speedo says I am going 35mph).

However, once or twice the entire gauge clauster has failed in me and I receive no data whatsoever.

Any ideas?

Mustang in front of the Tetons for your time