I have two new Amazon Fire Stick and have significant issues with buffering of Live TV. I don’t have a problem in playing back recorded programs; just live TV.
Check your settings menu for “enable fast TV startup” If it is checked (on) uncheck. This will add a few seconds to your channel startup but could help the buffering issue. I use the Firesticks also, had slight buffering problems with the fast startup enabled, seldom see buffering without it enabled.