As said could be any number of things to name a few, carbon build up on intake manifold flaps, injectorss, fuel filter, maf, diverter valve, crankcase breather valve, etc etc. Best way would be to first get it scanned to see if any codes. Then maybe get it put on vcds,vagcom by someone with it and do some measurements to see what maybe out. Maybe have a word with a vw specialist in your area that may have come across it before.