leefish

it me again leefish this time my problem is a misfire or some thing ,thought it was dirty petrol as i live in a vert dodgy part of ireland ,along the border and we get a lot of fuel laundering and for those that dont know what this is, its mixing petrol with heating oil. most petrol stations have some sort of scam going on with fuel so it rife here..well back to the car my fiesta 1.4 zetec usually going slow with small revs i get amiss ,bit of a chug , like driving at 30 mph then put the foot down you get a small pull back, any ideas, had new plugs fitted some months ago after the trouble with the watter coming in got that fixed.