I suggest that you turn off each computer or disconnect the
Ethernet for testing. See if choppiness remains. If not, turn
on/reconnect each computer and test for choppiness. If it does,
suspect that something on that computer is consuming excess
bandwidth (probably upstream). If you don't see the choppiness,
start applications that you had running previously. Servers, such
as torrent applications, and viruses that you do not yet know about
are possibilities.
You made changes in your modem, such as setting up QOS or
special SIP settings. Try reversing those changes.
Is your problem worst about 10 pm weekdays?
Pinging, particularly over a long period, can be useful. Pinging
your OBi, Pinging your SIP server if you know that, and pinging
something independent, such as 8.8.4.4, continually can be useful.
Check the recent pings whenever you experience choppiness to see if
you see a correlation.
http://www.pingtest.net/ can be useful.