Not getting the speed I am buying and Cox blames my modem.

Not getting the speed I am buying. I recently upgraded to Ultimate and am not getting the download speed I should be getting. Should be 300 down and 30 up. I am getting the 30 up but only 100 down. My modem is fully rated for that speed and beyond but Cox blames it on my equipment. They say that are delivering what I should be getting. Modem is an Arris SB 6183. Our neighborhood has been plagued with slowdowns almost every day. No answer for that either. Arghhh....