My 2010 had stuttering issues on shifts which is well documented here... specifically with Unleashed and LMS... the car would go to shift under WOT and it would stutter before catching the gear.. sometimes it wouldnt catch gear until very late if I pushed through it. Topping off a pint over full helped a lot but it never fully solved the issue..never was able to figure it out..
Hammering it while turning almost always produced the issue.. think trying to swing the rear end around into a drift.. it just flat out wouldnt catch gear and it wasn't the advancetrac kicking in, it was a mechanical issue.
I hope you are able to solve it, it was a major PIA at the track.. some runs would go flawlessly and others I would have to back out halfway down the track.. it would do it most on the 1st to 2nd, but occasionally on other gears..
That 2010 had been beat to death...so I am SURE that played a factor.