Man, you've almost got me stumped here. I don't see why changing your local IP address would make a difference.
The fact that it only happens with ooVoo makes me wounder if its a problem on their side. I know that every so often my ooVoo will log out for no reason at all. But ooVoo knows about this and is working on it.
Well I'm going to suggest 2 more things, one try upgrading your router firmware. If you are having IP problems for whatever reason, that could very well be the problem. Type your model number in Google along with "firmware" and see what you can find. Or go directly to Dlinks website. Just make sure when you flash the routers firmware that you only do while plugged directly in, ( which you seem to be ) and DO NOT turn off the power, or you will brick the thing.
2nd, give ooVoo's tech support a quick email to find out if its a problem on their side. Let me know how it goes. Also have you tried signing up again? have you tried doing it in IE just for the heck of it?
I'll also make sure to forward this to the billing department :D