I'm at a disadvantage, just reading symptoms on a computer screen.
Try drill-turning the cable (connected to the speedo) from the transmission end of things. Next try drill-turning the speedo on the bench. This will isolate if the problem is in the cluster or the cable. If in the cluster, then the only thing I could recommend is clean & lube the workings. Maybe a speedo shop can work some magic. Aside from that, sounds like replacement time. New speedos go for $125. I got an old working odometer at a swap meet for $15.
Not much, but all I've got. Good luck!
Bill
