thats funny... now that you mention it, i think i know why i clench and unclench my jaw all the time. its coming and going all by itself, i will have to try actively looking for it. it could well be that it comes with inner-retreat-mode.
clench/unclench, or clench left/clench right for all the years... gave me a muscular jaw that made the assistant at my dentist shiver when he said "now bite... hard"
i guess, too, that something to relieve stress physically, in an agressive way, might work for you. if you find a really tough stressball (or similar), let me know
I think I'm going to have a look on the internet because I rarely go to shops.