I am of the opinion that driving it fast and hard is always better. But I think you may be right that the fluid is dirty and the mesh is getting clogged. But there is only one way of knowing for sure and that is draining and changing the fluid. Swap out the solenoid that are suggested and inspect the mesh. Bavarian Auto sells the filter kit for your transmission. Not sure about the solenoids. Good luck.

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2010 X5M /// R63 AMG...Wifey's 540HP Family Hualer

2001 X5 3.0i Sport 335,448 miles & counting...
SMG M3 Steering wheel
E46 Stainless Racing Headers
4.6is Exhaust
Turner Pulleys
Afe Intake
Brembo drilled rotors
PowerFlex Bushings
Hualingan
30mm H&R Spacers rear
20mm H&R Spacers front
Bavarian Autosport High-Performance Coils
Active Autowerke's Performance Software