was having exactly the same issues with my transmission and I did the rebuild then the problem came back again after three months.. I ended up replacing the complete transmission with a used one from an identical car with 90,000km

My problem was caused by the heat exchanger (gearbox cooler) which got faulty and mixed up radiator water with gearbox oil.

Good luck with your tranny... I know how annoying this issue is!

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BMW 1M 2012 - Current
Jeep Grand Cherokee Limited 2011 - Current
BMW E93 330i Convertible 2009 - Sold
BMW E46 320i 2004 - Sold
BMW X5 4.4i 2002 - Sold
VW Golf GTI 2000 - Sold
BMW 740i 1996 - Sold