DPF or Something related with White puff smoke

[Somersetting]

Hi, I posted a while back that I had emission issues and they were solved by installing a new DPF and general clean of Injectors. So MOT pass and happy. However and let me be clear this does not happen all the time, the exhaust emits a thick white puff of smoke nearly every time i use it. There is no real way to describe when it does it as in when you start it or have been driving on overrun a while going down a hill or sitting in traffic or on very long runs. It just does it randomly, There are no fault codes and the van has been given the right soft ware relating to the Label on the door pillar and runs quietly with no tapping. But this emission burst of puff of smoke cannot be right. Any pointers gratefully received.. Note i have faith on the Mechanics as they are baffled as much as I am but obviously showing them would mean they would need to drive around until it happens and that is not going to happen.