Read the forum guidelines
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....
SB6183 is fine for Ultimate. It sounds like you are hitting a 10/100Mbps bottleneck somewhere. If you have a router between the modem and your computer, make the router AND your PC has 10/100/1000Mbps or Gigabit ports. Also make sure you are using quality CAT5e, or CAT6 ethernet. Some CAT5e can do gigabit, some can't.
As for "slowdowns", can you give more info? Do you mean slower then 100Mbps? Or do you mean the upload? Network congestion IS a thing, but I wouldn't expect it to slow you down that much. Try posting your signal levels. If your signal is too low, or even too high, natural fluctuation of the signal as lines warm/cool from the sun can cause issues. It's a tiny deviation, but if your signal levels are on the cusp, it can matter. See here for instructions getting signal levels from the modem. Basically, enter 192.168.100.1 into a web browser.