DT I know you are bound and determined to remedy this problem, but this just seems like one of those X3's that cannot be fixed.
This "Surge" and Lag you report is text book AT 07 problems. If they offer a Trade Assist, take it.
I drove more 3 series and 5 series in one year than I can recall and the reason I was unrelenting with BMWNA (besides the obvious) was that the sedans I drove never exhibited these issues and the 3 Series used the same GM6 transmission. So the explanation that they all drove this way, never held any weight with me.
2013 S4 Loaded (and NO RFT's) Awesome is an understatement
2008 MT X3
RIP you were a faithful sidekick and will be missed.
2007 AT X3 RIPOS
2000 Jeep Cherokee
1997 Twin Turbo RX-7
1984 GTI Wolfsburg Edition Neuspeed and more