had the same problems with my old vivaro, loss of power etc, I changed the air flow meter and crankshaft sensor and fuel filter and it was fine.
If I was you I would change the fuel filter first then open the old one up, If you have metal filings in there then your fuel pump is on the way out. (common problem with those pumps) If there is metal in your filter all your injectors will need replacing as well as pump, cheaper to get another engine or another van.