If it was my bike first thing I would do is replace the clutch plates. I have had 2 or 3 bikes have the same issues with 1st to 2nd and like you only from time to time. And all other gears were fine. And every time I replaced the clutch plates as it was the cheapest place to start and every time it fixed the problem. Or the other issue it could be is the dogs on the gears are worn and at certain points it can make the gears jump out or not go in right. It�s a common fault on the cbr 929/954 and zx6r. It happens if a bike has been raged and kicked into gear at high rpms or just poor gear changing. I would start with the clutch first. Hope this helps a bit. But sure there are a lot of people on here that may know more. Kev would be the best one to ask as he is the man on these bikes