I have been experiencing a similar problem while trying to A/V iChat. I am in NYC with a cable modem connection. But instead of 6.5 minutes, after 2 minutes and 14 seconds the audio feed of the person I was chatting with would drop off to zero. They could still hear and see me, for a few minutes sometimes. Needless to say, one way conversations get kind of boring.
I have a Netgear Router, and a Vonage VoIP box between the computer and the cable modem. If I remove the router and the Vonage I have no problems, but then I don't have phone service or the router. I moved the Vonage to the other side of the router, meaning that it no longer between the router and the cable modem. This is not what Vonage suggests, but it still seems to be working. And now my iChat problems seem to have gone away. I hope.
Opening up ports on the router did not help at all.
Vonage is fantastic, but I didn't expect these problems with it. There must be some process that it goes through every 2:15 that was causing this problem.
My troubleshooting suggestion to anyone with this kind of problem is to remove any routers or other devices between your broadband connection and computer.
If you want audio only, Skype is now out for the Mac, and that works great across platforms. And nothing seems to bother it from makinig a connection. But it lacks the video interface of iChat, and the userbase of AIM.