Agreed, likely the lag issue but do check which drive logic mode you are in. I have found that if you are in 4 or 5, the throttle blip when downshifting can cause a feeling that the car is surging. Especially if you are expecting some engine braking with the downshift!
2011 Space Grey E90 M3, DCT, Comp. Pkg.
1997 Z3 2.8, Artic Silver, Tanin Red interior, MT
1974 2002tii, Inka, 5 sp manual