I switched to Comcast some time ago and I've had a ton of trouble with it. Compared to my DSL, it's blazing fast... when it works. For months now my internet just decides to turn off every now and then. The length of time it doesn't work and how often it cuts in and out can be anything and has been everything. I've had it turn off for a minute and sometimes for days. I've had it turn off every 2 minutes for 30 seconds and I've had it perfect for weeks. This never happened with my DSL, but it was so sluggish that even this incredibly unreliable cable is still better. I've tested the problem and every time my computer (which is running Vista Home Premium) and my Xbox have told me that it can't contact the DNS. Resetting my modem works, but not reliably (at all). When I contacted Comcast their representative told me that since I don't use the modem they sell they can't help me with the service that I bought from them. I'm using a Motorola SB5101 Surfboard Modem. Comcast also told me it could be my router's problem. But I'm positive it's not. My computers and Xbox can communicate with each other on my network via my router. I've got no idea what to do at this point. Any help would be greatly appreciated.

It could still be your router but the first thing I would do is replace your Ethernet cables connecting your computer to the router, and the one between the router and the modem. These $.25 cords are not very robust.

You might try bypassing your router and connecting your PC directly to the modem (you will have to reboot everything to pick up the IP assigned by the modem).

Finally, I am on my 2nd SB5101 because my first started cutting out in a similar fashion as yours.

I've replaced all the cables, including my coaxial cables as well. At this point I'm thinking I need to replace my modem but I don't want to get a new modem just for the problem to persist.

I agree. That's why you need to eliminate the router from the equation to be sure it is not that.