Mine did this when I used castrol 4T at 4000 miles changed to amsoil and problem disappeared.
I use Amsoil too. It still does it when I get around 3-4000 miles on the fluid. It's not noticeable under normal riding, just like the OP says, when you are hard on it, it will slip. change the fluid it will be fine again. I'm looking at adding the Aim variable press plate.