Bouncin' speedo

[Griffon Financial]

I have a 91 200 20v tq with 156k and it all-of-a-sudden the speedo has intermittently started bouncing when I'm driving. It will bounce 5mph+ ( 50-55) Is this something electrical or mechanical? How can I fix it ......?    
            
                                                                                                                                                 Thanks 
                                                                                                                                                                Tim       
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