Trans problem - VSS??? 98 windstar

98 windstar trans - 120,000 mi
The van would not shift when cold. It locked up in second. Speedo would not work. After 3 - 5 miles all worked fine and car ran all day
without problems.
I had VSS changed. Trans shop called to tell me that car ran fine for 10 minutes then trans is erratic. This is exactly opposite of what was wrong before. He wants to go into trans and see if a gear is broken. Min $750 for next step. He said it could require more work on top of that. I have used this shop in the past under different ownership.
Does this make sense? Could he have ruined something?
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