It's possible that the IPv6 on the router if it's enabled is possibly causing an issue. I presume that the ISP and ISP modem are not supporting IPv6 protocol yet?
I have seen some issues with Windows Vista and 7, being that IPv6 is enabled by default causing some minor issues. I usually disable them unless there is a need for it. There has been some issues with Advanced DNS services as well. Why I recommend not using it and using ISP DNS IP addresses along with DNS Relay. Less processing by the router if we can help it. Just use the features of the router that you really need kinda thing.
Also does this happen with one specific browser or others? Opera, FF, Flock?
It would be interesting to see if a different cable modem does the same thing. Need to narrow down and weed out the causes if possible. Can the ISP identify any logs from there side and see if there is an issue. I just had to call my ISP as we were down for a bit. Was there issue I guess. So they can tell you if there is any issues, down time info and errors seen, if you can get a good tech on the phone.