I own a 2004 VW passat B5.5 1.8T. I have just changed the cat. because the the old one broke. I also cleaned the throttle body and replaced the gasket.
As i started the car it would idle just fine at 800 rpm, however when i drove it out of the garage and shut it off to clear the P0420 code caused by the bad cat. it would jump up and down in rpm from 800 to 600 and eventually stall, when i startede it again. As long as i keep it above 1000 rpm or drive then theres no problem until i let go of the gas and hit idle rpm again.
I have no error codes/ engine light