Sounds like you are losing hydraulic pressure within the transmission hence causing the slips.Your seals might be going or some of the valves might be sticking sometimes.

How many miles?

Do you have the right recommended atf in the tranny?

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)