Sounds also like it could be an issue with the crappy Comcast equipment-for the record I have had Comcast for quite a while now and have had ZERO issues with them. I would STRONGLY recommend purchasing your own modem and router (2 SEPARATE devices) to alleviate the issue. Not only will this save you the rental fee that Comcast charges monthly (anywhere from $7.50-$25.00 USD depending on service and devices rented) it also gets rid of the ability for Comcast to jack up your router for their uses.
Modems-the Motorola Surfboard SB6141 (you can find on ebay for about $65-$85+shipping) is a very good DOCSIS 3.0 modem (capable of faster speeds + higher speed boost+ better all-around throughput).
Router-I personally use (and recommend) Cisco Home (formerly Linksys) products my router is currently a Cisco EA4500 (wireless N with USB input at router) look into getting a good (preferably dual band) Wireless N or Wireless AC router.
Also as far as speedtesting goes do NOT use the Comcast (or any other ISP) provided test as it is specifically set to use their own servers and will produce false results. Instead use
www.speedtest.net-also for modem and router research as well as general internet complaints browse over to
http://www.dslreports.com/
not only can they help with specific product recommendations and the like their forums are also actively monitored by tech agents from the major ISPs who can (and will) take proactive actions to help their customers (such as line maintenance) without you having to scream at them.
For DIRECT COMCAST support go to:
https://secure.dslreports.com/forum/comcastdirect
For RECOMMENDED SPEEDTEST links go to:
https://secure.dslreports.com/faq/13860