gemmy michael myers help

I'd make another video of him without the costume on if possible. Closeup shots on the mechanism. Like you said something might of slipped or perhaps been stripped or worn down. I'd turn the volume down if possible so others can listen to the mechanism. I'm assuming there will be a bunch of plastic cogs inside. Are there any on strings? Got to see inside and close up.

no volume control on this prop

Well listen with your ears near the mechanism. Try not to become a victim. Listen for rattling sounds. I don't know the lingo but if there is only one motor then it obviously has to be interconnected to create the multiple movements. I remember a similar prop that got sent back to me that would not perform a similar movement because the dye-cast metal part has broke in two. There was like a high tension point and it just gave way. I think it's going to be something like this. Have to see pictures of the inner workings though. Others who know way more about mechanics than me would need to as well.

Well if the motor is working and doesn't sound like it's laboring too much that wouldn't be the problem. I'm just a lay person so it's hard to see what's going on with such a close up shot. Those cogs could be the problem but you'd be able to see if they were worn. I think there is another part that possibly broke. I'm not hip to the terms but look around for some sort of lever hanging loose or a metal post that may be snapped. If something is snapped you don't find the brokn piec but you may see where it broke off. Moving the torso would be the most stressful part of the operation. There has to be some kind of tension point away from the center to allow this. I'm just theorizing but look around for the outer edge for something that may have snapped, like a small metal post.