french hobart n50

good evening to all

i have repainted my first hobart n50, reassembled and used it, and all is well.

i have a second hobart n50 which i have also dismantled and repainted but there is a big problem, which i can't find.

it works, but after a few gear changes it starts spinning in a vacuum.

it feels like it's coming loose somewhere inside.

I've taken it apart once, thinking the problem was solved, but it keeps going.

has anyone ever had this problem when reassembling?

thanks

je prefere parler en français si possible...