You're a little bit beyond where I was, but at the first hint of a shifting problem, I switched my 87 300D over to Mobile 1 synthetic ATF. That cleaned it out and its shifting fine. I expect this was due to varnish and sludge buildup in the valve body, where tight toleranced pistons have to move back and forth inside bores.

Your problem MIGHT be the same, but more advanced. Did this come on suddenly, or has it been developing for awhile?

Compared to a transmission overhaul bill, I'm convinced running synth ATF is the way to go even though double the cost of regular ATF.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles