What you're describing is quite common, actually. My 540it had the 2-1 downshift lurch when I bought it last year, mostly because the seller kept the AC off. Once I turned the AC on I never experienced that shifting issue ever again. I keep my AC on 24/7/365 so that's a permanent fix for me.

I'm sure if I went outside and drove my 4.6 with the AC off I'd get the same downshift lurch despite having a rebuilt transmission with a new torque converter.

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2003 X5 4.6 | 2000 540i Touring

Formerly owned:
2004 Range Rover | 2000 M5 | 2000 540i/6
1995 750il | 2003 Mercedes S500 | 2003 540i/6