newbie speedo question

Hi Spridgeteers!

My new baby, a 1960 Bugeye, has an annoying habit:  The speedo will jump and
swing wildly while in 4th.  It will settle down for a few minutes, but then
go off again. Do I have a short?  Has this happened to anyone else?  (I
would check the archive, but I can't seem to get through - so my apologies
if this has come up before)

Thanks,
Kevin "a newbie spridgeteer"
Chicago, IL