So I called my mom to find out the deal. Apparently, MTS (my ISP) came down on Tuesday of last week and found the problem: the hot water tank was leaking, and caused a bunch of corrosion inside one of the walls. He removed that one jack that was shorting out (the room was unused anyways) and that solved the line noise problem.
That's likely the reason that the Internet connection was being intermittently broken last week, so hopefully it'll behave now for awhile.