No amplifier...Comcast came out last time (visit #10) and did signal tests on all 3 TV's and modem and all were within parameters. They charged me for the visit finding nothing wrong. Ha...funny.

This time I have video of this issue.

They replaced every line in the house, even the service line to house.

The only thing I did outside of Comcast tech recommendation was to unplug the box to #2 to swap with the bad one (but didn't do the swap) and now the issue is apparently resolved.

Only thing I can think of...By unplugging the box on the good TV but not swapping it reset that box solving the issue?