I have a 00 Passat 1.8t 51K that is driving us nuts. When you first start it up cold, it will stumble around & misfire for about 30sec to 1 min. then instantly clear up and run perfect. This is what I've tryed, new bosch plugs(from dealer), temp sensor, We also did an secondary air pump because it wasn't working (water got in it) It seemed related because as soon as you hear the secondary air pump shut down it seems to run fine. I unhooked the power to the pump & it still does the same thing. We get multiable misfire codes & long term fuel trim system to lean, all the vaccum hoses look good. We gave up & took it to the local Vw dealership. They had it for 1 day and de-carboned it for about $200 bucks with some fancy machine and said its fixed. Went to leave, same thing. They had it for about 2 weeks and tried all kinds of stuff, leak down test, ecm, and more that did not work. I got fed up because the bill was aready over $400 and its no different. So we picked it. They said they had another one doing the same thing, so I figured let them expeirment & spend their $$. I called them 2 weeks later to see what they found & they said they de carboned it 3 times till it cleared up. So we bought 2, three step kits that you put in the fuel, throttle body & intake(that stuff stinks!!) Drove 2 tanks through it and still no good. I am ready to take the car to an auction and say byby. Any help??? For awhile it did run good on startup but we still got the system to lean code after a while of driving. But now its back to its old self. Thanks for your input!!!