My 06 YZ450F is having problems shifting from 3rd to 4th. During a race it gets stuck and will not shift into 4th gear. I can stand up and put all the pressure in the world on it, but it will not shift. Then after trying for a while sometimes it shifts. I bought the bike brand new. I had the same problem about a month after getting it. I adjusted the clutch and it seemed to go away. Now it is back. I have always used yamalube. I recently changed to amsoil. It seems to be worse when the bike is warm and under power. If the bike is cold I can start it up and go around slow and it shifts fine. Also, I can shift it fine on a stand. When I take off at the start and go into 3rd it is fine, but then going to 4th it sticks. I kept thinking is was something I was doing, but I have been riding it for 2 years now and not had the problem except once right after I got it. I read the thread about a bearing being bad and changing it out to and 07 one. I am going to check that and do that. Does anyone else have any suggestions of what it could be?