Audi A3 1.8T surging problem?

I have a 1999 A3 1.8 Turbo. The problem with it is, (especially in 3rd) gear when I've floored it for acceleration, and lift off the gas, the car lurches forward. I've had it at Audi agents, and the only error codes they found were battery disconnect, and temp sensor. I replaced the sensor, and just in case, the lower suspension mountings, also the dump valve. It doesn't always do this, but when it does, it seems to do it just as the turbo has fully spooled up and is boosting. It now has no error codes now, but still does this, but not every time. The catalytic converter was cut out and replaced with a straight pipe, and that's pretty much all that has been done to it. What could the problem be?