Well, let's start off with something simple.
First, did you try uninstalling the item pack(s) all together and reinstalling it?
Two, are you sure you put everything in the right place? Including all the meshes in the items folder, etc?
Three, sadly, it could just be your computer if it's lagging out. Best if you try to clean some unneeded stuff out of your computer, maybe, or run a virus scan just in case. Of course if it's an old computer... Nothing I can do other than suggest a different computer. owo' I'm pretty sure this wouldn't be the case, though.
Hope I could help somewhat, though.