This could be due to a neck issue. It might also be Masticatory myopathy which you can read about here
http://www.vetinfo4dogs.com/dhead.html#Masticatory%20myopathy%20possible
I would suggest seeing a veterinary neurologist for your best bet in diagnosis and treatment. This site might help you find one
http://www.acvim.org/Specialist/Search.aspx
or an internal medicine specialist for small animals.
This might be an autoimmune problem (not uncommon with thyroid issues) which could be treatable with steroids.
You may want to read here about facial nerve paresis
http://www.petplace.com/dogs/facial-nerve-paresis-paralysis-in-dogs/page1.aspx
You can read about the test here
http://www.veterinarypartner.com/Content.plx?P=PRINT&A=1755
It appears there are no false positives but about 15% of dogs show a false negative.
I think a specialist would be a good idea to pursue this problem with.
Hope this helps you!
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