I have a 300tdi with an auto box and when going uphill at over 65 she has a cough 3 or 4 times just like a misfire when it tries to change gear it also can do it on the flat at around 85 any ideas
I had a similar event occur to me whilst driving my Disco TD5 (i know you have said you have a 300tdi) back from Manchester to Surrey a couple of years back. When it went over 65ish it would sound like it was not running on all cylinders. Made for an interesting 300+ mile journey home.
Turned out to be a seized turbo waste gate. Could be the same problem you have.