its the same problem that me and my boys were having.
its the maf sensor related. the stock computer cannot see the boost, once it sees it thinks that somthing is wrong and put the car into limp mode(retard the timing). few things can be done, one of them is using missing link between the sensor and throttle body to release the pressure to blind the sensor so that com wont see the boost. OR you could use the s-afc to re set the map voltage setting. I dont know about your car but one the project car im doing, its suppose to be less then 3.0v but when it hits the boost it goes over and throw out the engine code and car goes into limp mode.
one your car, im sure its not using map sensor but something else, find out what it is, and same thing, measure the voltage and find out what it suppose to be. then reset the voltage or use missing link or check valve to correct the number or pressure to within its limit.
what engine is the turbo for? m42? if so how much boost are you using?
also did you upgrade the injector and pump? and whats controlling the fuel delivery system?