when the boost drops ( say under load , going up a hill ) if you lift your foot off the pedal momentairly then accelerate again , does the boost come up before dropping again . it sounds like this is what is happening . this is hitting limp mode . i am not sure but , i don't think a code is registered when this happens . i would suggest fitting a dawse and needle valve . search this site for info on these valves and also the blanking plates there is lots of pictures , diagrams and location information here that can help you understand and fit and adjust these items if you wish to do so . they are one of ,if not the most common mods that people make to zd motors . study up on these as when they are properly set up you can use them to fine tune them to have the car running the way you want . they are part of the nads system and most would suggest you apply this to your car .