rough downshift

I have a 94 towncar that downshifts roughty. For the first few miles it shifts normal and then starts to shift rough. Sometimes it down shifts at 60
to 70 miles per hour. It will shift rough as you slow at 35 to 40 miles.
I ran a computer code check and everything checked fine. I need suggestings on what might becausing the problem.
Thanks Jim Tierce

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