Ok, might need a little more info here as well.
Wired connection.. direct from the CGN3 to the PC?
(not using a powerline connector, etc?)
What type of cable are you using? Is it a Cat5e or 6?
And what type of network card in the end PC?
As well...
Can you log into the CGN3, and post the POWER LEVELS for the channels?
Should be 20 or so downstream and 3 upstream.