Can't help with an exact passat problem but my escort 1.4 did this and the cat was blocked,I manually "de catted" it and she was spot on,might be worth a look?mine sounded like it was hissing,no power and in the end it acually failed,just died on a hill.

I left it 5 mins and it started and it wouldn't hit 60,after the decat it saw 115!!!

Did tap a bit after that!!