We have an Iveco 75E17 Tector with a cold running problem?

Our 75E17 has developed a fault which no-one appears able to sort.

The truck starts fine and runs for about 20-30seconds then starts to 'hunt', usually stalling. Will start straight-away, but usually does it once more and then is fine. It has had 4 new injectors, new sender unit, we have tried by-passing the filter and primer, but the problem persistes. We think it is air in the fuel line, but no idea from where.