If it's the same engine found in a 2001 Peugeot Boxer diesel then from personal experience......fuel filter
May not sound right, but I had a company one in 2003 which did exactly what your describing. It was pouring out black smoke big time.
Turned out to be the fuel filter. Dealer was able to diagnose over the phone too so must be a common problem.
Got it towed into the dealer who fitted a new filter straight away and no more black smoke or loss of power. Was back on the road 10min later.