Sounds like you have a cracked drum and it can't retain the pressure until the engine is at 2500 rpm and then it slams into gear. Do a Google search for cracked drum transmission BMW.

Here's a post from another forum.

Bimmerfest - BMW Forums - View Single Post - BMW X5 Transmission Problems

Or it could be the o-ring



Here's a link to the website with the above picture. "WARNING" it's not a BM Trouble U site, but it was a BMW X5 trans tear down.

5HP24 home repair on a BMW X5. - Jaguar Forums - Jaguar Enthusiasts Forum

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