i was going to suggest looking at valves but you said you did.
honestly it sounds like it may be something out of your range of fixing if it's something with the gearbox. i would take it to a mechanic or do some google research/download the bike's service manual.
service manuals are pretty helpful, i've been able to fix a lot of stuff on my streetbike that way but when it comes to gearing i probably would just take it in because you don't want to mess up the clutch or the gears as they can get pretty expensive.