you need to get it plugged in and have the codes read (you can have fault codes without your CEL coming on) as sounds like you are going into limp mode which could be for any of numerous reasons. im sure someone more knowledgable will come along but im pretty sure all will say the same - get the thing to a vagcom and see what it says
mine does the same very infrequently, no CEL comes on, but when read vagcom says overboost. in my case i have a grand plan to rectify (change the remap completely) which needs a few extra mods before i go for it.