I have one of these running on Comcast right now and have had zero issues with it. Bought it through the same refurb deal a couple months ago. These originally had major issues staying connected but Comcast pushed through a firmware update for them that took care of that.
What firmware update would that have been? The most recent one I am aware of is dated May 14, 2012, (I believe) and there are many, many folks who are having connection issues with it. Check out the link someone (sorry, forgot his name) posted to the Comcast support forums a few messages up. But I really, really hope you are right.