Usually what you are seeing is caused by a faulty router or bad setup of said router, its possible Comcast is having issues, but its not plausible unless your neighbors are having the same problems. You may have an issue in your House wiring, but that is not Comcast's property, its yours.
can you post the modem stats, should be at 192.168.100.1 unless you have a dory [all in one internet, phone, & wireless], then its too hard to explain.