modem wifi

I have comcast: internet 200 mbps, voice and tv. My Gateway is the: 

Voice and Data Modem

ARRIS Group, Inc. TG1682GSerial #: 3C7A8A85FDEC

cast: internet 200 mbps, voice and tv. 

I've have had the modem for 3 years. I'm considering getting another modem. It seems my download is slow. When streaming I'm getting a lot of buffering. I'm not sure if I should get another gateway or seperate modem and router.  Also, when I connect with an ethernet cable the reception is not any better. I thought when you go strait ethernet that's the best way.