I've been fighting the same problem.  It turns out in my case to be
right at the end of the speedo cable, at the wheel.  The square portion
that pokes out through the grease cap has rounded itself.  My fix so far
is additional washers and then several wraps of wire to pull the cable
end up snug (part is still square).  Since it's working fine right now I
keep forgetting to order a new cable.