Custom rig jittering when inserting humanoid

Okay so my friend made some rigs for a game me and some guys are making and for some reason whenever we insert a humanoid into the rig, it starts jittering. If we group the hrp into a diff model from where humanoid is, it works which is really weird. I have tried creating collision groups for parts but that didn’t work either.

recoding of the glitch:

Thanks in advance!