Loss of power and engine shaking (leak off mod already done)

[Hanatarash]

Hi there,

First time poster from the Netherlands. We have a 2000 Mk 5 smiley campervan (Reimo) with the 2.5Di (non turbo) banana engine.

We're having some engine troubles. I've done some browsing on this forum and it looks like the problems we are experiencing are somewhat common in these engines.

While driving the engine will suddenly lose most of its power and start shaking. It won´t go past 80km/h (50mph) after that.
If I've done my research correcty then this probably has to do with air getting into the fuel system. I've checked and one of the previous owners has
already done the leak off mod.

Two questions.

1. The leak off mod has already been done. Could it still be that air is getting into the system or could it be something entirely different?
2. We're off to England and Wales for a month in july and I'm sure we'll encounter the issues along the way. Is there a 'quick fix' I can do when these issues arise? I realize that this is dependent on what the actual underlying problem is, but going on the assumption that it is because of air in the system.

[Hanatarash]

Funny story. As we speak, my wife is stranded with the van after the engine shaking issue reared its head again this morning. She parked it at work and after a few hours tried to start it again. The engine was still shaking and when hitting the throttle van stalls immediately. We haven´t experienced the stalling before.