general problems of the 2006 Ford E-150

I own a 2006 Ford f-150 4x4 pickup. Two weeks ago the speed odometer went crazy, it started out sticking at 10 mph and moved all the way over to 120+ and sometimes drops to the 40 mph area and then right back up to the 120 mph zone. I have never had this truck over 75 mph. . . Ever. I checked for a cable malfunction but I found out there is no cable, its electronic. This truck has 144,000 miles what is going on with Ford these days? is there a recall on something? how do you fix this? I have already changed all 8 coil packs on this truck for an unrelated problem with ignition. I hear the spark plugs are a pain also. All I know now when I drive this truck there is no way to tell how fast I am going. I just judge my speed by how fast everybody else is going. If nobody is on the road I am screwed. . . Thanks a lot Ford !!!.