i had the same EXACT problem on my busa..i took it in to the dealer since it was only 6 months old, and they found that the dogs were worn on second gear so it was jumping around when under a load.which is why they didn't cover it under warranty,but anyways yea,definitely a transmission problem and it needs to come out of the bike and replaced..i also had the shift forks replaced at the same time as a precaution