The Comcast IPv6 beta ended over a year ago. IPv6 is now deployed in most or all markets. The only thing that is coming up on the horizon is static IPv6.
What is the make and model of the modem you have?
Usually poor performance is a result of power levels on the cable side going off. You can try posting a screenshot of those from your modem.
One thing that (surprisingly) often can contribute to electronics doing strange things is a power supply failure (ac adapter "wall wart") If everything was peachy then suddenly went to pot then try another a/c adapter of the same voltage, and polarity that is equal or better current rating.