2001 1.8T (Auto) acceleration problem

When I come to a stop (say, at a stop light or stop sign...) in my 2001 GLS 1.8T automatic (34K miles) and then proceed to lightly accelerate (i.e., not stomp on the gas but accelerate lightly and smoothly...), the launch will be light and smooth for about one second, and then the car (and engine) will surge forward unexpectedly. I usually react to this by letting off of the gas pedal, which stops the surging. Then, upon re-depressing the pedal, the car accelerates as expected until I come to a complete stop again. Does anyone know what could be causing this (or has anyone else experienced this same phenomenon)? My car is at the dealer for the second time for this problem. The first time, they replaced some sensor (I forget which one...), but it didn't help. This time, they say they can't reproduce the problem (likely because they're stomping on the gas during their test drives...).