Car: 1990 420SEL

Symptom:

When the car is cold in the mornings it jerks/slips into gear ever so slightly when first on the road going from 1-2 and 2-3. Also, after about 2.5 miles on the back roads I get on the freeway. In the mornings it doesn't shift into 4th gear at all until I'm maybe 7-8 miles down the road. Economy needle is at the 1/2 way mark and RPMs are higher than usual. Car has reached operating temperature by this point.

Transmission works like a champ later in the day even after sitting out for extended periods of time.

Any thoughts? Is there a part which modulates it or is the transmission on its way out? The fluid was changed probably 40-50K miles ago and the mechanic noted then how clean the fluid was. Fluid is not low. Car has 198,700 miles and has the original transmission.

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1984 300 SD 384,000 Miles