Power loss suddenly from 80km/h ; 1.4 Tdi 75PS

Hello together, just drive the A2 1.4 Tdi 75hp from my uncle and have massive problems with the performance. After starting the vehicle and driving off, everything is good. The gears up to the fourth is still everything great. But suddenly from quite exactly 80km/h the line is suddenly gone and I hardly come up a mountain. Even if the speed falls again under 80km/h, the power stays away in all gears. Yesterday already from above and down to loose Look out for vacuum hoses, but couldn’t find anything.