is the device directly plugged into the unit (network cable right from the PC to the modem).. its not going through any wall jacks, etc through the house?
Which exact model do you have? WIll help in troubleshooting.
The Newer CGN3.. is actually kind of backwards to most of the industry... a yellow/orange connection is actually gigabit, and green is 10/100.
(so yellow is actually preferable)
That you were getting different between the different cables.. could partially be the difference between a cat 5 cable, and a cat5e/cat6 cable.
Have you tried swaping ports, moving it up to one of the ports that the other devices are into?
Wondering.. if a port has gone, on the PC or the modem maybe... getting a partial link, but nothing more.