Either that or your clutch is slipping in 5th gear and when you shift into 6th, the clutch catches up and lower the rpms too low so you have no power....on the street, there is no resistance, but the dirt has a lot of resistance and makes it hard to tell if the clutch is slipping or not...If not, then it could very well be that your jetting can be one or two sizes off like Fixitrod stated or your over geared, what size tires are you running w/ what size sprockets??