I'm not big on "miracle" additives, but in ATX cars I've owned a transmission additive called "Trans-X" (blue can) has worked surprisingly well. It has stopped or reduced slippage and delay engaging drive when cold. Maybe using this will squeeze some more life out of the tranny. I last used it on my old beater, an '88 Corsica w/230k (yes it's true!) that ran strong but experienced a long delay engaging drive (~1 min) and slippage. The delay and slippage disappeared and the car ran well until I sold it at 260k. Give it a shot.
