I had the same thing happen to me once, it turned out that my shifter was not adjusted just right, when I shifted to second it was not engaging all the way and letting the valve body bypass fluid. Under hard excelleration it would do the same as yours. If I was in the pits and went to second it would feel like I was applying the brakes. I was skeptical when my tranny gut told me that what is was but sure enough I adjusted it just a little and it worked fine. May not be what yours is but worth checking.