TL;DR - The Arris SB6183 cable modem is dropping IPv6 TCP traffic with the
timestamp TCP option. See the "Workarounds" section for possible workarounds.
13 months later (October 2016), my modem was updated to
D30CM-OSPREY-1.5.2.3-GA-01-NOSH. IPv6 tests show this problem is fixed with this