Thanks for using our Answers Forum. I understand that you are having trouble connecting to your Slingbox while on a comcast internet connection.
I do have a couple of questions for you. The first being, is this Comcast internet your home internet? The second, what exactly do you mean by
"won't work", what exactly is it doing or not doing?
Slingplayer for mobile devices gets poor video quality, either at home or away should provide some useful information.
Hope this helps.
Sling Moderation Team
It thinks since it's working initially, it's probably not a firewall issue, and more likely a bandwidth fluctuation issue.
Check this article: How to optimize WI-FI streaming performance
The article helps dial in things to give you the best possible streaming experience.
Hope this helps!