Chuffing and black smoke

[BZM]

57 plate Ford Transit 2.2 diesel

I have had the above Transit from new. It's always had this problem, and has been checked out twice at different Ford dealers. Neither could find a fault.

What happens is... When I go to accelerate away from roundabouts, or am pulling away from somewhere, the engine will hesitate - like a puffing - and will emit black smoke from the exhaust. It only ever does a single 'puff' at a time, so is over before anything registers. No lights come on on the dash, so I have no clue where to start looking for a problem.

A mate of mine suggested it might be an air sensor, or the EGR valve. Is there any way of finding this out without too much hassle?

Has anyone else experienced this? Id' really like to know what's causing it.

Thanks.