Idling problem with DAF 45 130 R Reg?

When idling after 4-5 seconds it starts to fade then picks up again for another 4-5 seconds then fades, holding the revs at approx. 800rpm same happens. It runs well at high speeds. I've replaced the fuel filter, by passed the water trap and replaced the lift pump but fault is still there.

John J
April 2010