The stopping does not appear at any given specific time. It will stop at random moments. I do hear that when it stops there is sometimes an audible pause in the beating (which will sometime resume on its own, sometimes not).
You might also have one hooked or otherwise malformed tooth on the escape wheel. In your pic, I think I can see one hooked tooth at the bottom. Look at each tooth on the EW and see if the end is bent over or broken.