What would cause my speedometer not to work on my 2001 ford windstar?

Sometimes the speedometer works fine and sometimes it goes up to 20/25mph. Then stops there. All other gauges work fine. I took it to the shop and they replaced a sensor and the whole new speedometer console thing and it still went up to 20/25 mph. And stopped so they put my original parts back on so I didn't have pay for them. They told me to take it to ford dealership. Any ideas will b appreciated Thanx

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