I had a Buick that did this. I put in some snake oil stuff called Transmission Fix and the problem went away. Made by Lucas Oil. Worked for me, just give a couple of weeks to work. Not sure what it does but it worked. It's been 3 years and I have not had a problem since in the car. It's like thick honey and U use half a liter or so, about half a quart bottle.

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1982 300SD