I was having issues with this as well when i converted to an avchd camera from a dslr. here is a thread that might shine some light.
https://www.newschoolers.com/ns/forums/readthread/thread_id/705579/
Also, I was using the workflow that i mention in that thread until recently i cut it in about half by getting a program called Wondershare video converter ultimate. It basically does the same thing as mpeg streamclip however it is able to actually read .mts files and you dont have to re-wrap them with clipwrap.
hopefully some of this helps.