Turbo Quattro Coupe Wastegate

[bruceh@garlic.com (Bruce Hochuli (Ventana Computer Corp.))]

am looking for help in two areas.

1.) When I turn off my engine after about 30 minutes of
    freeway speeds, it will either 'fart' out the tail
    pipe or let out a mighty backfire. Since I have
    just had a new turbo installed, I am paranoid
    about damaging it. My mechanic, who has gained
    a lot of expertise on this car, thinks the cause
    is a worn wastegate. Any comments? Does this sound
    realistic?

2.) Where are repair parts (kits?) available for this
    repair? My local dealer will gladly sell me a 
    whole new wastegate for $1200.00 and I am of the
    opinion that theree must be a better cheaper way
    to do this. A secondary reluctance to part with
    my current wastegate is that is has been modified
    for adjustable boost. This modification has been in 
    place for 9 years so I don't think it caused the
    problem. When ABT was in Southern California, they
    sold 'European' spec. wastegates and their own
    adjustable boost kits for way less than $1200.00.

Thanks for any information and it is good to be in 
contact with this group again.