make sure you are shifting completely. This bike has a longish throw from 1-2. Even with my reverse shift I have missed 2 and ended up in neutral.
Double check your clutch adjustment, make sure you are not partially pulling on your clutch assy. It could be slipping a tad and you dont notice it, then any pressure on the shift lever will make it fall out of gear.
Other than those two issues I dont know. These transmision are pretty rock solid, shift like a freight train, but they can take some abuse. Look into the above and see if that takes care of your issue. Next step is to drain your oil and look for metal residue, or have the oil sampled and see if there is any metal deposits in it. But I would bet its your shifting or clutch.
DJ