I've been chasing down this problem for a year now. Immediately after launch (at WOT) it will shift to 2nd, then immediately 3rd. After a second or two it will downshift to 2nd and then continue the run. Here's an example of a pass I made last night:
After WOT, shift to 2nd at 4400rpm, 16mph.
Shift to 3rd at 4500rpm, 24mph.
Downshift to 2nd at 4800rpm, 37mph.
I'll work on posting the tune and the log of that pass. The only other issue I see occuring is about 4* of KR right at the launch (which I'm pretty sure it attributed to a banging exhaust), but that wouldn't cause it to immediately shift, would it?