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I had the same exact issue, surging while WOT in first few gears and surging at 100+ mph when WOT. I would get SES and reduced power lights all the time because of this.
I tried the upper intake cleaning, did not help. I tried replacing the spark plugs, car idled smoother but still the surging was still there. I ran BMW approved fuel system cleaner through the car, still did not help. Finally after 3months of frustration I figured out what it was...
I had upgraded to the lastest software on my JB4 G5 ISO board which requires that you rip two small diodes off of the JB4 board. (instructions should be stickyed around here somewhere). Once i pulled those two suckers off the car is as smooth as butter, have not had one single SES light or reduced power light. From what I understand, the JB4 and ECU are fighting for control over the turbos and causing them to blow off pressure while still under acceleration. When you rip off the diodes you are preventing this conflict from happening. Just my experience, i hope this helps!
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