Just tried your steps 1-5 with my Express 4K+. Still same problem which may be two fold:
in addition to the picture freezing after about one minute into a show while the audio continues, I have also noticed three things on login:
1) When the blue light comes on indicating power I am noticing that a quick red blink in the blue light also flashes once after about 5 seconds of powering on.
2) The R O K U logo letters no longer appear when the TV is turned on.
3) I'm also getting a TV device message saying there is no signal, but in fact the signal is OK and strong when checked in settings.