Hi Had exactly the same problem and it was the mounting for the vacume pipe on the big plastic pipe which goes from the inlet to the intercooler which had snapped off. Next to the battery and in front of the fuel filter ther should be two vacume pipes coming off, one from a sensor and the other to the turbo waste gate. It had me fooled for ages as the snapped off mounting just looked like a rivet and the vacume pipe was dangling out off site at the side off the battery. Interesting how lifting the throttle and putting it back down restores power, but what a pain in the ass when towing with a caravan on a motorway hill!!!!! Hope this helps