Had same AVCS prob on my 03 STI after ECUTEC reflash and tune from Maximum Motorsport took it back to Steve from Maximum and he plugged in Delta Dash and we went for a run and it showed up no advance in the cam timing under hard throttle but all else was ok, so car suffered from loss of midrange.
He was able to fix this prob with the AVCS and has had this prob before with other STI's cost of fix was under $200 but he wouldnt shed any light on what the cause was.
Never suffered from jerky prob just down on power.
Hope this is of some help, if not maybe someone else may ring a bell.
Steve.