The same thing happens with the automatic tranny in my car...seems to bang back down to a lower gear and slow speeds...kinda like when I am driving from stop sign to stop sign. It pushes me in my seat. When you find out what it is, please let me know...as my dealer has stated that "this is within normal GM specifications." I haven't gotten any codes though, but it bangs really hard to the lower gear, usually from 3rd to 2nd or 2nd to 1st.