Power Loss?

I have an L reg Daf45 that has developed a intermittent loss of power early in a journey.

Power will be lost but the engine not stall the first time you pull up a hill. Dip the clutch...come down the gears and power comes back.....

Once this initial problem is overcome then truck runs fine.

Thought initialy it was dirt so changed the tank and the filter.... then suspected the lift pump but problem still persists

any Ideas?