when I first saw this i thought you had a problem with an Alfa Montreal speedo!
So an auto S4? Not so sure you have a speedo cable on that one (someone know for sure?)
AngloSpider, a member in the UK had problems with his auto S4 speedo. The auto speedo sender is different to the manual sender, and you cannot buy it anymore....never seen them s/h either to be honest!
What you have is an impulse generator sender screwed directly into the right hand side of the gearbox, that sends a pulse to the speedo. On an auto spider you have in any case to drop the auto transmission down somewhat (as if you were going to replace the rear gearbox mount) to replace it.
Might be worth getting under there and at least checking the wires are intact and not chafing somewhere.
On the manual spider which also has a speed pulse generator fed by a speedo cable from the gearbox, then wires from th sender go directly to the Speedometer itself, going through fuse #7 (7th fuse from the left in the pull down fuse box)
I would check that in case the auto sensor runs through the same fuse (don't see why it shouldn't)....pull the fuse and clean the contact or replace.
Photo from the manual shows the location.
This link shows what it looks like as well as what it looks like inside, when Anglo got through gently opening it