Android tethering via bluetooth keeps disconnecting
I'm doing the exact same thing, except with my Captivate, and I ran into the same problems. To solve them, I use three apps across the two devices. On my N7, I use Tasker to detect when I lose wifi connection and use Bluetooth Auto Connect as a Tasker plugin to automatically connect to my phone. On my phone, I use Bluetooth Auto Tethering, which automatically turns on Bluetooth tethering when you start your phone.

As a bonus here is a link to my Tasker profile/tasks which does all this stuff. I think you can just import it into your Tasker.