I had a very similar problem as well... as soon as I got to 80mph my X5 would kinda shut down and would not produce any more horse power. I took it into the shop and they found two problems. They smoke tested the intake system and found a vacuum leak in one of the elbows. Cheap fix like $18-25. They also tested the MAF sensor and it was not giving the correct reading. I drove for another 1000 miles or so and the problem would come and go. So I replaced the MAF and problem was solved, she ran like a champ after that. Hope this helps.

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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