2004 Buick Rendezvous Spedometer going haywire

The spedometer on my Rendezvous is definitely malfunctioning. When I drive long distances (over 50 miles) the spedometer will suddenly shoot up to where it appears I am going 120mph when I'm actually going about 65-70, or it will start to decrease and show that I am only going around 35 mph. I have been at a complete standstill and the spedometer shows that I am at 70 mph. The only way that I have found to reset it is to turn the ignition to 'accessory' numerous times, over and over again, until it zeros out. Any solution would be greatly appreciated!