I had the Same problem, DSG (BKD) 2.0 TDI.
After VW, Replaced All four Injector Seals, Rebuild/ Recalibrated the injectors. Replaced Starter Motor. With no fault codes found what so ever.
I would be looking into MAF and or Temp Sensor. I would also advice to clean the EGR.
I did not have my car long enough to rectify MAF, Temp sensor or EGR. As they could not get my DSG sorted. So returned the car in the end.
As for testing temp sensor. It's pretty easy but you do need VCDS.
When the car is stone cold. Have a look at the temp sensor reading in VCDS, If it's not the same temp or less than ambient temp. It requires replacement.
Would be great to hear the resolution to this. As I really want to get back into a TDI.