help needed with hijet fuel problem


hi everyone, has anybody got or had the same problem as i have?I own a 998 cc hijet, whilst driving the thing starts to stutter (like running out of petrol) put the thing out of gear and cruise abit then jump it back to life, goes for a while then the same problem, have changed the fuel fillter, and just to be on the safe size placed another (plastic type) up by the carb, none of which seem to be full of sh*t after a run, is there a fillter in the tank of so sort that could be getting blocked?, or could it be something else? that I,ve over looked ,the van drives like a dream sometimes, this tends to happen on motorways at around 60mph or more, any sugestions?