Are there any dtc's.
The output shaft speed sensor on the transmission sends the signal to the cluster for the speedometer.
Does it fluctuate with vehicle sitting still, as well as when driving?
This would be an indicator of rfi interference withing the wiring, routed near a plug wire or magnetic field.
Another way to descern it as rfi, when driving and speedo is erratic, put vehicle in neutral and shut off. Does speedometer return to normal or is it still erratic. If it returns normal, then this is your problem. If it is still erratic, I would be more inclined with other problems.
Hopefully this bit helps. Find out exactly when it is happening and lets go from there.