erratic speedometer 3,8/96 - what to check?

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lately I got occasionally jumping indications at ~60+, going up and down around 10mph. As this is a mechanical cable, question would be, what usually goes sour at over 100ts miles.
Is it worth while taking all apart or is there a known weak spot in the system?

Fortunatly I have the heads off, but I am not so enthusiastic to take the dashboard out too ...and like to order all parts in one paket...