Am not a seasoned Audi owner; had my 2001 A4 Quattro 1.5 years and turbo replaced at 40k miles due to oil sludge failure. Like new turbo's performance but feel somehow that accelleration is variable when punched from start or low speeds. Good when I get aggressive response; not so good other times. Seems consistent when switched to Tiptronic, however. Possible cause?