Leyland DAF 45 130 (pre LF) running slow?

I have a R reg Leyland DAF 45 130 Euro 2 pre LF horse box, I am having problems with loss of power, The engine in all gears sticks at about 15000revs (the bottom of the green bit on the rev counter), I have changed the fuel filter, checked the air filter and drained the sediment trap on the fuel tank and this at first appeared to do the trick, a few days later and the problem has returned, although i did manage to get above 15000 revs when i started the engine but the revs dropped off, a friend has suggested changing the fule lift pump. the enginre runs fine and the truck drives alright, no smoke from the exhaust, no misfires or anything. its a bit embarrassing going everywhere at 20-30mph!