My X5 (2001 4.4) now has about 104k miles, and is still going strong as an ox. As I stated in April, my X has been doing the weird RPM drop occasionally since I bought the X with 85K miles.

I haven't noticed the problem getting any worse over the past 20k miles. I am almost certain that, at least in my case, it is not a vaccuum or fuel delivery problem. It does it so randomly, and never does it in Sport mode, which still leads me to believe that it is a tranny software/control issue.

I've moved on from worrying about this issue.

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Now:
E53 4.4i Sport
E39 /// M5
back then:
E36
E30 (x2)
E21