Fiesta Off Throttle

My first time here, my great little car needs a fix but dealer can't find problem.

So it's a 2.1/2 year 1.6 TDCi Titanium which is great except that it isn't happy when I take my foot off the gas............it just won't run smoothly for some unknown reason, my dealer has had it and checked all they can think of and can't find anything.

Occasionally it will die on me for a split second, I liken it to a person struggling for breath........this only happens once every 2-3 days. There have been a couple of times when the engine speed has increased with no help from me at all.......neither of these things are daily events but the rough running when I'm off throttle is, for instance in a 30 mph limit in 3rd gear it is all over the place.

It's controllable for now but it isn't right, anyone had the same experience ?

Yes it's under warranty but the dealer doesn't seem to agree with my findings so I'm looking for any help I can get to take back to them.

Thanks !