Piaggio Porter 1.4D Engine holding back

Hello everyone!

Sorry for the long text, but I want to describe the situation as good as possible  

I live in the Netherlands and since 9 months I am the owner of this Piaggio Porter 1.4 Diesel from 2003. I love the van a lot and have used it as a everyday car for the first 7 months to drive to work without any problems.
Recently I got the idea to convert it to a micro camper and do a big trip to eastern Europe and maybe even further.


To prepare the van for the trip, I brought it to the garage to do some work on it. Some things that have been done are:
-New oils and coolant
-Changed the diesel tank with a second hand one from Italy. (old tank had a lot of rust on top and a broken cable to the gauge).
-Changed diesel filter
-New brakes and partially new break pipes

Unfortunately the van has started to show some problems recently. On my way to Germany, after 4 hours of driving, it started to stutter lightly. Gradually it started to stutter a lot more and it lost power. The engine started to hold back a lot and seemed like it wasn't getting diesel. To a point that I had to shift back to first gear and couldn't drive faster than 5 km/h. I went to the side of the road and had to really do my best to keep the engine running.
After waiting for around 15 minutes, I started the car again and drove off. 10 km later, same thing happens and the engine dies. At this point the engine didn't want to start anymore. Startmotor functioned well.

I called roadside assistance. The mechanic found that there was a valve next to the filter that had a loose hanging connector.


He changed that connector, but it back again and bleeded the diesel system.
The car again started, drove for 10-15 km and had the same problem again with stuttering very badly and kind of choking and not having power.

After that, I got a towing company to tow the van to a garage, where they said they didn't have enough time to fix it in the coming days, but after an inspection they suspected for 95% that it was the diesel pump.
I ordered a new original diesel pump from Italy and got another garage to mount it together with a new diesel filter.
While mounting the new pump, the mechanic noticed that one of the hoses on the old pump was not tightened properly which could cause it to suck in air and cause an airblock, explaining the problems.


Nevertheless we decided to mount the new pump anyway and tighten the connections firmly.
He then bleeded the diesel system and the van started perfectly.

I did a long test drive to a nearby city (25km) and after driving perfectly, on the way back it experienced the exact same problems. Stuttering, engine holding back, no power to a point that the engine wants to die.
I stopped, turned off the engine, waited a bit, started normally and drove off. Again after 10km same problem.

I have no clue what could cause the problem and hope someone here can help me out.

Other tips that I got which didn't work are:
-Taking off the fuel tank tank cap (someone said it could possible create vacuum.)
-Taking off the one way venting valve on the tank. (again possible vacuum)
-Diesel injection cleaner

Things checked and looked good:
-Air filter
-Valve next to the filter (clicks when on contact)

Thanks a lot for taking the time to read