I don't know if it's just me, but automatics and loud exhausts just don't mix.
I don't know what flat sticking is either.. but if I think it's what you're doing (moving the gear selector up and down manually selecting gears excessivly), you are damaging the linkages between the gear selector and the gearbox itself. Autoboxes like that are meant to be left in D, not to be treated as if they were tiptronic gearboxes. That's just my opinion.