issue is in the Subject line. Doesn't appear to be an easy way to communicate with an actual person at xfinity so I'm hoping that they monitor these msg boards so we can submit a support ticket.
What do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.
What is the exact make and model number of the modem ?
Is this a WiFi connection ?