One additional test to validate that it's the ISP and not your end......if you take your computer to another network (NOT Comcast) and have the same issues, that would lead me to believe there might be a problem at your end - if there are no issues displayed while on the other network, then that points very clearly to a Comcast network issue.
The problem COULD be hardware at your end, but it would be THEIR hardware, not yours.
Give it a try and let us know.
