VeryKen...the exact same thing was happening to me...my modem just nosedived in april after many years of service...MANY...my tests here showed me when it happened...and i knew it was my modem because of a decreased signal that my isp was seeing...about the same time people started asking why the difference in numbers between test sites...so i checked out a few and most showed the numbers i pay for and better...only here at this site was the problem detected. i replaced my modem with a new motorola sb6120 same as yours...and bingo back in business.
so bottom line is if people want the fluff and vanity sizing then go to the other sites...but for those that aren't afraid of the truth and really want to diagnose the problem...come here and test and post up in the forum what seems to be ailing your machine and all the free "doctors" here will help ya make her all better