Mine does the same, checked the motor and all seemed fine. - I find on high speed they move further accross the screen. My firsth thought was that the motor was knackered, but there seems to be plenty of power in it when disconected from the cable.
What i think is happening is that the coiled cable that connects the motor and blades is stretching and compressing causing the arch that they sweep to be reduced and generaly **** wiping.
May just change the whole lot (motor, cable spindles etc) for a newer version as its the same motor my 1973 MG midget has on it !)