Check to make sure your drivers for your NIC are all up-to-date.
Turn off your pc, and modem for 15-20 min for a refresh from you to comcast.
Turn your modem on first, wait for all lights to come back on, then turn your pc back on.
Try running a virus, spyware, malware check. Those could be messing with your internet too.
If those don't work, contact comcast..