model getting messy or black screen (2 problems)

[Worpertesk]

https://youtu.be/Xd0HD7augGk

so all what i wanted to do is to make a model for ken, i wanted to attach a new upper body to him, but if u watch the video then u see that my model became a mess, how to fix this?

and sometimes when loading the cutscene the screen stuck at black screen for no reason, i wonder if something simillar happened to u and u found a way to fix both of these issues..