Hey J, bienvenidos a Bimmerfest! It sounds like you are taking your car to mechanics who work on BMWs, and not BMW dealership mechanics, right? There is a lot of information, called proprietary information, which the average mechanic cannot access without a special scanner. Your engine computer stores a lot of clues about what is happening at the same instant that a CEL (check engine light) is set. Getting at those clues will help determine the cause of the problem.
Here is what I think is going on. Your turbos are capable of making a lot more boost than is safe to use. At a given boost level a sensor triggers a solenoid which sends vacuum to a device which bleeds off the extra boost. There is something wrong in that process and as soon as the computer senses it it cuts off the boost to save the engine from damage. That's why your engine loses power.
Every time you start your car the computer checks to see if there is a problem in that circuit. If no problem is detected it allows your turbo to work like it is designed to work. Your engine then runs OK until the glitch is detected again.
So what is causing the glitch? Well that's what good mechanics get paid to find out. No way any of us can do that for you. (Unless I head back to Rincon again in the near future, but not likely!)