I have a similar problem, but mine is a 2.0i petrol engine with 25,000 miles, and it's freaking me out
I have a JB+ and a K&N air intake, so my first step is to remove the JB+ and go back to OEM air intake. Then, the dealer will update the software for all the modules (transmission, etc.). And after that, make a full diagnosis.
The spark plugs were replaced 5,000 miles ago, so I think it could be the air sensor, transmission, or fuel pump (maybe?). What else could it be?
Have anyone experience this issue with a petrol engine?
Thanks!