I've been running a 2.0 GTTDI MK5 Golf for the last couple of months and every so often it's been running rough, misfiring or not boosting as intended and the mpg has been rubbish.
I plugged the car into diagnostic equipment and found nothing and continued to experience the problem. It seemed to be more in the rain but I couldn't tell for sure.
I looked up the symptoms and saw the regularly occuring problem with the injectors or injector harness but the misfire was different and more apparent between 1750-2000rpm and it has never cut out on me.
After a few weeks of randomly experiencing the problem the car for a split second brought up a visit workshop code which erased immediately form the dash.
I plugged the car in again and found P0238 Turbocharger Boost Sensor A Circuit High Input (568 Short Circuit to B+).
This lead me to the MAP sensor which turned out to be a cheap manufacturer. I replaced the item with a genuine Bosch MAP sensor and it has behaved ever since.