misfiring type symptons on mk5 d.i

[shippers]

this mite hav been asked a million times before but as im new to site n this is my first transit can anyone help, ive got a X-reg mk5 2.5d.i non turbo with 117k on it, 9 times out of ten it runs lovely n i dont hav a problem but every time the fuel gauge gets down to quarter of a tank it starts like a misfiring type problem, that seems like the only way to describe it, i put a fuel filter on it a month ago and some redex diesel treatment thru it n it ran lovely, i stopped using the local BP fuel station as i thought it could be down to the more modern bio-fuel it was stating they sold on the pumps, and started using the esso garage but it seems to have carried on everytime the fuel gauge drops to a quarte of a tank, but once u top up a tenna or more it seems to cough n splutter for a mile or so then cure itself HELP quick please as its getting realy annoying ?????

[MinorMatt]

2 suggestions... one free and one cheap

Next time it starts coughing and splutting take the fuel cap off - if this solves it the tank breather is blocked

The second thing is there are 2 ports on top of the injectors - either the front or the rear has a blanking nipple, which is it?