I am guessing (just a guess), that the jaw problem you are experiencing could be caused by Upper Motor Neuron damage. These problems usually manifest as brisk reflexes (reflexes that happen to healthy people and are normal and go away in matter of just a few seconds but not when you have UMN dysfunction) , when there is UMN damage they are multiplied thousands of times (clonus, jaw jerk, startle reflex, etc.).
I'm not saying that necessarily that's the cause for what your are experiencing but, you should check with your Doctors just in case.
Hope this helps.