Hi Dave:
In the 14 years I've had Comcast internet service I've had 5 modems start to fail and usually the symptoms are either periodic lock ups or very slow internet. I would start by power cycling your equipment to see if your problem clears up for a time after that. If that's the case then just take the modem to a Comcast office and get a replacement.
If you are using a router try and bypass the router and go stright to the modem, I've had one router go bad and cause the kind of thing you described. If it's better going straight to the modem then your router may be starting to fail.
- Norm