I have had this problem for ages and finally managed to get to the btm of it but it took alot of Saturday mornings at a friends Garage . I was getting intermittant boost and never full power, turn off ignition back on and away i went until it happened again. Usually only happened in 4th or 5th. On my Galaxy the turbo wastegate wan't functioning fully, was eitherr only 80% close or open cant remember. These are prone to sticking too, Lubed it up with a bit of WD through vacum pipe and manually moved the vane rod. I also replaced all vacum hoses. Some were quite brittle, well worth doing as a small pin hole can cause the same symptoms. If the boost control valve is sticky lube it again. U can check this by pulling off the vacun hose and suck u should hear the valve move inside. Brake servo hose can get brittle and cause a loss of vacum. Mine was broken right at the servo just feel around it for any shap edges. Do you have any fault codes? I was getting a MAP fault Manifold Air Pressure, was told it might be the ECU (didn't want to pay for that) but the faulty wastegate causes the same fault code. Hope this helps My Galaxy now pulls like a train, never been so good Its a '98 TDI 110BHP