Slipping out of gear is usually caused by worn dogs and windows in the gears or maybe a bent shift fork and not going into gear all the way. When you did the update were the parts new or used?
WELCOME TO THE FORUM.
Thank you for the welcome.
If I remember correctly, and this has been 12 years ago, the 02 gear set was used, but like new. The dogs and windows were square. The shift forks and shift drum were suppose to be all the same. It did it from the first time I started it and I never tore it back apart to see what the cause was because I couldn't find a head and cylinder gasket for my big bore kit; I probably could now if I went looking.