2004 WR450f with the motor basically stock.
I was out ridding today and suddenly I had a problem with third gear. I can shift easy into and out of third gear. It stays in gear until I put a bit of power to it and then it feels like it just goes to neutral. Then when I let off the power it catches again. It only happens in third gear. It is not the rear slipping or the clutch as I can nail it in any other gear and no slipping. With the motor off putting it into third and rolling it turns the motor over so there is some engagement.
Any ideas what this could be? Is there any chance of fixing it without splitting the case?