Transit 2.5d '94 Intermittent power loss during long journey's? ?

But it seems ok on shorter 1's (like round town). If i'm doing 60 ish for more than 20mins, thats when it starts to die. I have to take my foot off the throttle for say 5 secs, then when i put it down again i get a bit more of a acceleration. I repeat this saga til i get to my destination...I'v recently changed the fuel filter (and had all the associated problems with that-van not starting etc. but i filled it with fuel which sorted that), but the van had the fault b4 the change-which is why i changed it. Any ideas? p.s. Is there meant to be an arrow on the fuel filter and on the housng to the fuel filter? Why would there be this? The filter only goes on one way??
Anyhow, thanks-Ben.