Corsa B 1l 12v has a judder when driving

[MontyC]

A friend has a corsa B 1L 12v that when you accelerate lightly whilst driving it jolts and hesitates it only does it whilst your going along not when stationary and not if you accelerate hard either. I have changed the air mass sensor which has made no difference, my next thought was coil pack but surely that would make things worse if you booted it as its under more load but thats not the case, any ideas it also seems very noisey when you first start it from cold but im not sure if thats normal being 3 cylinder?