Looks like you are right - put a 3TB powered drive in and things seem fine. Shame the Pi can't handle more power but I don't suppose plugging USB HDDs into each USB port was a design consideration.
It's a "grey zone" between powered enough and no enough power. The current consumption not "flat", it's dinamic, and sometimes, for short time, can be insufficient, leading to strange issues. I had some strange freezing issues with my RPi3B, after changed the power source, no more freezing...
And why only the picture freezing... probably it's something with media player or the codec, when in the stream some issue occurred, sometimes the video part can't be recovered, remaining blocked, meantime the audio part continued... that can explain why with a skip forward the video started working again.