This sounds like the reverse servo valve in the valve body is sticking and comes free with increased RPM and feed oil pressure. If it engages with a bang and no further slippage is evident then you are well on the way to cooking the clutch pack. Take the advice of the other guys. Get it checked now. Benzmac is right. This kind of thing only gets worse.

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Jeff L
1987 300e
1989 300e
1987 BMW 325