Hi,
We've had a major gearbox problem this year that involves 2nd gear just jumping (slipping) into neutral by itself, we even disconnected the solemnoids of the pneumatic shifting system we run (since 2009) to see if the solemnoid was activating by itself but still slips out of 2nd gear to neutral.
I looked around and found out that this problem usually occurs on CBR600 (thats the engine we use) when the 2nd gear cogs are old. So we open up the engine and found out that actually the edges of the internal gear, or counter-shaft gear were badly beveled.
So we change up the gear shafts for another one we had in other engine and this time it started to slip 2nd after only 2 day usage.
So we obviously decided its nothing related to the gearbox, but something external that wears the gears up to the point that it makes 2nd gear slip.
We have some things in mind about the cause of the problem, one of the things that looks most convincing its that the pneumatic system does not have enough power to fully engage the gears, so after time it wears them out. So the only difference between 09,10 and 11 car its the lever arm length, and the 11 lever was lot smaller than previous.
Thanks for your time