I just went through this with my kart. I was getting a false neutral in 5th gear and the motor would zing and not put down power. There are two different main shafts for the different years of a cr125, I thought I had the wrong one but this wasn’t the case. If you search this site for “5th” and the old ekarting news forum you can find the part numbers. Take a closer look at your gears again, I thought mine looked pretty good but they were indeed rounded on the dogs. It was expensive to fix but no issues with the new pieces installed. Personally, I think there is too much gear play on the secondary shaft and thought about adding a shim to help hold it in gear but decided against that. Check that there is a lot of space for the shift rod next to your seat too, I saw marks on my seat and it was flexing in fast lefts and pushing on the shift rod while on power (bad).