Sticky Speedo

[flyfisher 6 2 Cars]

comeing back from the brighton meet the other week the speedo on my 205 mi16 came to a sudden halt at 65 mph it would not move further. but on tapping the perspexs cover did fall back .. since its done this several other times. just wondered has anyone else had this. i put it down to a faulty gauge thats sticking. but could this be anything else any idears