Its pretty common for cable driven digital dashes to jump around.
Every car I've ever had has done it, all have done it at different speeds too.
Its one of the main reasons why I put a UZZ30 dash in my Z10, to get rid of the speedo cable.
As for the gunk, every one I've seen has had a fair amount of gunk in there, but without seeing a pic of yours its hard to say if you have more than usual.
I see no reason why it can't be cleaned though, as long as some grease remains in there.
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