I should also mention that I notice a bit of a clunk when slowing down to a stop, when the trans downshifts. I've read that this can be caused by a vacuum leak? Perhaps a vac leak or a bad trans modulator is causing this, and the slipping?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles