Macbook Air / IPv6 Sinking Home Network?

Macbook Air, OSX 10.8.1, on a home network under a Cisco/Linksys WRT110 router, internet via Optimum cable modem TM822G. Other PCs are W7 and XP, plus an iPad, iPhone & droid.

This mixed setup had worked fine until Optimum (Cablevision) replaced our aging cable modem. Since then, whenever the Macbook attempts to connect to the network it kicks all others offline. The Macbook itself can't get online. The network created by the router remains OK, but the connection to the cable modem & internet is lost.A couple of years ago, when a W7 computer was first introduced, it had the same effect, and we determined that turning off IPv6 on the W7 computer resolved it. But since then, we got a new router and it had no problem with IPv6. I mention it only because the behavior is the same. But in this case, it's a new cable modem that was introduced. Now the Macbook Air, which has been working fine on the network, crashes internet.Unfortunately, the Airbook is a high school issued piece, so we can't turn off the IPv6, due to access restrictions; not that that would've necessarily done it, but it's hard to troubleshoot when you can't make changes.So the two pieces involved in the problem are the two pieces we have little control over.