Sometimes a cassette witll run too fast if the pinch roller is not
pinching the tape against the capstan.
Look for a mechanical reason. The pinch roller could have just popped
out of place.

This is a known problem with some of Sony's servo-controlled portable
cassette decks. The D5 was prone to it too. As I recall, it was caused
by an open pickup coil in the motor assembly, causing the servo to run
wild.

Most likely, that's the problem.

Michael