I'm still looking to find the cause of OP's issues.
Since his signals are great (16 down and 4 up perfectly in-spec channels) and his Comcast on-net speed tests are good (95% of his provisioned speed), and the issue persists after he bought his own modem, I don't believe the issue lies with his premises or equipment.
I'm looking beyond his premises, which takes time, because I have to work with teams at the local level.