VW Transporter..

I have. Needed some egr cleaner and a blast down the motorway.

If the engine losses power at the same time my first place to start is to check the intercooler and its pipe work.
If the pipe/intercooler is leaking especially under boost conditions, the engine gets too much fuel and not enough air, as the air under boost is escaping back into the atmosphere, and the engine does see it. .
Lots of black smoke sometimes in big clumps, sometimes throws a fault code too much boost on vw, or it may not even throw a fault code.
Good through check of the induction system may be all it needs.might be a split somewhere.

I'd still stick a can of good quality egr cleaner down there. If you don't give the engine a good rev while the old egr is off the new one can easily get clogged up again.

It's a 2000 I think, not sure for deffo, has his own plate on it. Bio diesel doesn't burn well at all mate

The ones we had problems with were 2005ish so later than your one.