Flat spot on 330d

Hi everyone driving on the motorway the car seemed to have a flat spot in 6th gear doing 70mph. It was sort of like I let my foot of the accelerator for a split second. And also the sane thing happened at about 40mph. It never seems to happen when I'm accelerating but when I'm at a constant spend. It doesn't happen all the time either just the odd occasion.



If anyone has any suggestions as to what it could bic it would be a big help



Thanks Matt