I can't remember the exact name, but there is a vacuum line running from the manifold to the transmission, IIRC it's a modulator valve, designed to 'cushion' the harshness of the shifts. See if it's disconnected or has a crack in the rubber connector at either end. It's adjustable too....do a search, many posts on this.

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1986 560SL
2002 Toyota Camry
1993 Lexus