With your added power and 21's the tire diameter and over-all function (small side-wall) will be my best guess. What is crazy-making is that it's not all cars! There are other little random shifting quirks (worse with more power) too and all I have been able to find as a common link is the wheels. Not to say we cant figure it out though and we have on a few all working together.

Can you get us your current tire pressures and sizes?I have been able to

 

f: 255/30/21 psi:38 r:325/25/21 psi:44

 

OK try 40 and 42 cold to help balance the diameter. The rears are a little larger plus have less compliance. The car senses it's not rolling it's calculated diameter. Also you can re-learn it a little on adaptations. Try shifting early a few times with the new pressures. That may at least help. Keep us posted.

 

thank you for the information and I will keep you posted. Many thanks!

 

Happened to me once or twice. I didn't think much of it. These cars are so complex that I fully expect a few lines of code to get crossed up here and there.

- Patrick