Do you really mean at fast forward type speeds? Cuz then you do have another problem, one that I'm not sure of how to fix.
Lessseee....9191....if memory serves, there's a circuit board with a pot on it, behind the front cassette area, that controls the speed...or do I have it confused with another deck? Hmm.... well, if there isn't a small circuit board with a pot on it, then the speed is adjusted right at the motor, on the back of the motor, look for a hole with a screw-slot recessed in it. That's your speed adj.
Assuming, of course, that its' just running a little fast, and not racing across the head in FF as you suggest.