I just bought an 03 yz450f and took it out for the first time today. In the begining of the day when I would get the bike into fourth gear and riding at high rpm's the engine would rev real high every 5-10 seconds, as if I was pulling in the clutch real quick then releasing it. By the end of the day every time I tried putting it into fourth it would immediately do it so I kept it out of that gear and ran it in third. What do ya think, clutch problem or something in the gears or....? Any help would be appreciated, thanks.