Comcast, can throtte data from specfic hostnames, and have been accused of doing so in relation to Netlfix in the past.  I am not sure how much bandwidth you are using per month, but there have been stories of downloads getting progressivly slower the further over the 'cap' you are.

In general, in the past 10+ years of helping people whom have Comcast, I have in 90% of cases found the issue to be either with Comcast equipment, or it's network.  There is a reason it wins bad customer service awards.