vw t5 2004 1.9tdi goes into limp mode at 60mph

hi, the scanner readout suggests "boost pressure fault". no lights on dash. so far the garage has 1. cleaned the turbo 2.new MAF 3.checked high pressure/vacuum pipes 4. changed N75 5. cleaned EGR valve i am at a loss now, has anybody got any suggestions what to do next ? if i stay under 60mph...