I had the same problem a few weeks ago it got so bad I had to pull-up at the roadside ..I got a breakdown truck to get me home the take the car to their garage ... We talked about possibly EGR valve or worst scenario the turbo ... I got a phone call next day from the garage to say car was ready for collection ...the problem was a loose pipe on turbo cost for job was for2 jubilee clips £40 plus £60 for a 30 mile tow in the car now is running like new .