My car started to slip around 1 month ago after replacing the sealing sleeve of Mechatronic and added 2 liter of transmission fluid at dealer. However, the slip does not come all the time, only when I was trying to speed up sometime. Before having warning text of 'trans. fail save mode' in the cluster, a very weird sound and hesitation to kick down appears. Now, everytime I hear the sound and feel the hesitation, I just leave off the gas pedal immidiately to stop accelerating and press it again, everything returns to normal without having warning, but if I keep stepping on the gas pedal, the warning sound with the text will come and the system goes into safe mode. If the warning comes, I switch off the engine and start it again, everything returns fine. It also works fine when in manual mode. In my case, what the problem could be? Do I need to have the transmission rebuilt or what? Thanks for help! Here is the diagnosis result.

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.