Solved.
The problem was not with Buffalo. It was with my Netgear Nighthawk router. After FW upgrades and debug, we isolated the issue to IP assignments. The firewall router was setup for a XXX.XXX.2.1 address for years, different than default. After a FW upgrade, we got locked out of using the XXX.XXX.2.1 setup. After many different attempts, we setup our router to use the default XXX.XXX.1.1 setup after our 5th factory reset.
We were able to download the 4GB file cleanly and a more consistent network response with this setup. As a test, we reset to XXX.XXX.2.1 and had major issues with even logging into the router. Once established, downloads remote would fail at 1.5-1.7GB. The FW upgrade really made the router touchy.
For some reason, Netgear does not like for you to modify a base IP setup outside of XXX.XXX.1.1 with their latest software.
Good luck to anyone else that has this issue.