I’m working on a project and using roblox to model a spider and it’s movements in a physics enabled environment and ran across two rather frustrating bugs (at least one of them is a bug)This is normal movement (sorry not sure how to make viewable GIFS) And this is after random amounts of time As you can see the center base part stops bouncing up and down like it does normally.
Wow, that’s a lot of bugs.
On a serious note, maybe you should include a few lines of code so we can better understand this unwanted behavior.
All it is is rotation of welds no special code just some for loops and hard setting the cframes. I don’t think putting my code in here would help at all. 
also figured out that I’m an idiot for the second part. edited it out of the OP
Parts tend to ‘sleep’ when characters aren’t nearby. To test if this is actually the case, try doing
somePlayer.Character = Workspace.spiderModelAh, would this apply even when there are no characters ingame? IE pressing the play button?