The entire point of DNS is so that if Comcast changes thier IP address your DNS server account will change accordingly real-time and seamlessly make things happen on your end.

Have you reserved your static IP for your XP-8 on the router? Are the correct ports open? By reserving the static/Mac of your XP-8 may help things and keep things more stable. If something is knocking you off the challenge would be to find it. Firewalls, other devices that are using those ports and other applications could be a culprit. Last resort, ensure your DNS service is compatible with Comcast.