Animation bug help

Hi Im doing an animation and Im having problems with a character that keeps shaking in a wierd way without me animating it.
You can see it here

Any ideas on what may be causing this and how to fix it?

Im using the CG cookie flex rig, but both characters are the same rig and the other one has no problems.
Please help, I only have a couple of days to finish this animation.

the blend file:

Selecting the Root bone of the Barry.lowRes_proxy armature and then setting the Copy Scale constraint influence to zero and then moving it back to 1 seemed to remove the stutter

Thanks Richard it ended up working after trying a bunch of other stuff.
You can see the finished animation if you are interested: