Speedometer jumps between high and low states

Our 2000 Windstar (95k) began having intermittent (analogue) speedometer problems. It would jump between reading correctly and reading about 60% of actual (55 mph vs. 30 mph when going 55 mph). It would stay in one state for a few minutes or longer or would fluctuate rapidly.

Now it is stuck in the low state; odometer progresses slowly, too.
<<edit: needle is actually very steady now, unlike jumping needle others on the forum mention -- it's just WRONG -- 30 mph when going 55 mph>>

A repair place replaced the vehicle speed sensor (broke off a piece of the existing one and had to do some major disassembly to complete the job... MANY hours of labor for what they said would be a half-hour job).
Didn't work -- still doing the same. Now they think it could be a problem with the computer... sounds EXPENSIVE to me.

Any thoughts? Would be greatly appreciated...

If the speed sensor is good.....it could be the speedometer or possibly the speedometer cable.
I do not see how the computer would have anything to do with it.
The Speed sensor gives an electrical signal to the computer.
It sounds like you need to try another shop.