I have good news everyone! A complete draining of the transmission and torque converter and filter replacement with a Mann filter seem to have done the job! So far, the car is consistently shifting into 4th even when I drive it after the engine is hot and has been sitting for a few minutes. I put another bottle of Trans-X in with the new fluid for good measure. The old filter looked pretty dirty even though it only had about 25000 miles on it.

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1987 300SDL