Cable connection Issues

last week I notice a severe slowdown in my cable connection. At first I thought it was the websites. I noticed it mostly while watching things on youtube. I used to be able to set thing to 1080p and not have anything buffer. Now anything above 480 buffers several times. Netflix has also taken a hit. I took my router out of the equation and still had the issue. after talking to Cox support they say I need a new modem. I have a Scientific Atlanta 2100. I tried everything I know to fix it. Still I am getting 5M to 10M down during peak and non peak times. I am getting 10m up which is what I am supposed to get but I am paying for 50m down. basically I am looking into maybe just getting a new gateway and the 2 cox provides are the Cisco DPC3825 or the Netear CG3000D. I do not have a lot of experience with different brands. So I am asking what brand would you recomend? Is a gateway a good way to go or should I stick with just a modem and router? 

On a side note the router I have might be dated it is a netgear wndr3300. I am wondering if I just get a new modem with docsis 3.0 would it bottle neck it? I am still learning networking things. Especially WiFi. as of now I have 1 laptops,my desktop, our Wii, one tablet, a PSP, and 2 smartphones connected to my network. At most the Wii, desktop and tablet will be running at the same time, usually netflix. If I am going to be spendind money I want to make sure I am getting the most out of it. I also do not want to spend to much. 

sorry I tend to over explain I just want to make sure I do not leave anything out. 

They could be throttling... and that Scientific Atlanta stuff ISP's provide you are crap. I always recommend having you own cable modem, least then you OWN it and no extra equipment fees...

ya, they were throttling. After going up several tiers of support turns out they were doing so cause of, get this. 65 cents that was overdue. appearently I forgot to add that in when I paid my bill. Cox can suck my well cock. Doing something like that for 65 cents. But, I bet it is just automatic. Still dick move, but I have no real option but them. it is just like this