2.5TD Performance problems

Hello, I’m still quite new here. I’ve only been following the forum occasionally so far. But now I probably need some help. I have a 2.5TD, Omega B Caravan as you can see. For years I’ve had the problem that the performance seems to me often far too low. As soon as I reach the highway 140-150km/h, the performance suddenly collapses slightly. Obviously electronically regulated. I reach the level highway 140, it’s a bit steep I can also I’m not a racer, and I would still be able to do that if I didn’t have too little power to accelerate. Only a few days a year he prefers to be relatively normal. Otherwise, it seems a lot weaker. So far, nobody could help me. Just gave me some premonitions, like a problem with a sensor or the injection pump. Now I ask around here: is the problem somehow known? :/