You either have a cut split ring or the outer clutch guide isn't bottomed out in the fibers.
The stalling in gear makes me think its the later.
When installing the outer clutch guide (the gear in the center of the clutch) i install the guide first, then the clutch fibers/steels. This way the guide is guaranteed to be installed far enough in. If you measure the distance between the guide and the end of for mainshaft there would be atleast 23mm.
I've attached a picture of what I'm discussing.