I have a '99 A6 wagon that has been great until now. Recently it has started stalling while braking to turn or come to a stop. It started yesterday a few times and has increased in frequency today. At stop lights I have to keep my foot on the gas to keep it from stalling. Does anyone have any idea what this could be?

try replacing your fuel filter. being 9 years old i would guess that is the likely culprit. Other than that, it could be a screen on your fuel pump, or some sort of idle controller. I would start with the fuel filter first.