Why is my vivvaro smoking?

I have a 2002 vauxhaul vivvaro 1.9dti with 105k on the clock.
From 1000 to 2000 rpm it tends to feel like its shudering a bit and not much power.
When it gets past 2000 rpm its fine but does kick out some black smoke.
This doesnt happen all the time and more often than not it doesnt do it. On a previous van (kangoo) this was happening and i had the catalitic converter replaced which sorted it out!
I dont know much about engines so simple descriptions would be appreciated!!
Just want to sort this before it gets bad/damages the van

thanks in advance