I had a similar issue on my VDO's odometer flaking out at low speed and at idle. Turned out to be a loose connection at the inductive pickup on the diff. (mine does not use a trans sender)
http://www.vdo-gauges.com/media/inst...ter%28c%29.pdf
If your sender is attached to the Trans gear its a Hall Effect unit and that IS a failure symptom of those.
There is a shop in Austin that can repair your Speedo if you don't have that issue.
Overseas Speedometer | Speedometer & Odometer Repairs