1997 A3 1.8 erratic speedo
 

I have just purchased this car today, and on the way home the speedo would either shoot up or drop back down to zero, if you keep driving it will go back up again, this seems to be linked in when you decelerate. What could it be. It was bought as a first car for my 18 year old daughter, and I dont want her stranded anywhere. This car was bought privatley so there is no warranty.