Check your modem's specs against this matrix. I recently upgraded my modem, a $40 unit that was only about a year old, because it was removed from the supported modems list. I ordered this one, and have been seeing regular speeds of about 85down/12up on a 75/15 plan. http://www.amazon.com/NETGEAR-Certified-Comcast-XFINITY-CM500-100NAS/dp/B00R92CEVU?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00
Yes. Second this. Get a Router / Modem combo from amazon. I have the same issue, this fixed me up. https://www.amazon.com/NETGEAR-Certified-Comcast-XFINITY-CM500-100NAS/dp/B00R92CEVU
You might want to consider a separate modem and router. When one fails you don't have to replace both.
I use this modern and it works good for less than 500mb. https://www.amazon.com/NETGEAR-CM500-100NAS-CM500-Cable-Modem/dp/B00R92CEVU
Plus whatever modem fits your needs and budget.