Rather than replacing the trans. fluid, assuming that it is clean and smells o.k., try a can of Gunk brand, "Transmission Medic". It is available at Pep Boys for under $3. It did a good job of helping my transmission that had similar problems. This is not a sealer, it is more of a cleaner.

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'76 240D-Sold
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07 SL550, 14K Miles