I've had the 920 for two weeks. My first phone had battery issues and bluetooth issues, so I got a new one Wednesday and the performance is much better. Bluetooth is still not perfect but it is better than my first phone. I've been doing some semi-serious troubleshooting with the bluetooth and think I've figured a few things out. Hopefully these tips will help until we get an update from Microsoft.
I have a 2010 VW GTI that doesn't have very fancy features, just hands free calls (no phonebook, text audio) and music. I only use the hands free phone. I think there is an updated version of VW's bluetooth, so I don't know if these tips will help all VW (Audi maybe) owners.
I highly recommend removing any paired devices from the car's memory. I had to dig out the owner's manual to figure out how to do this. Mine is done by holding the phone button down on the steering wheel for 10 seconds or pushing the bluetooth button on the overhead console for 10 seconds. For me, it required that the car be running. It wasn't enough to just turn the key to get power to the radio. I don't know if this is mandatory, but I now connect more often than before doing this.
After getting paired, I recommend not turning off bluetooth in the phone settings. I was doing it because I was having battery issues and was trying to save all the juice I could. Now that I have a new phone I don't need to do that. What I found out was if I turn off bluetooth for a while and turn it back on before getting in my car, it would not connect automatically. I also found out that if I do turn it off for a while, turn it back on, and then reboot, it will connect automatically.
So to summarize...
Delete paired devices from car
Don't turn off bluetooth
If you do turn off bluetooth, reboot after turning it back on.
If you still have problems with consistent connectivity, I recommend replacing your phone (1 year warranty). This isn't a perfect solution but it keeps me sane until Microsoft can fix it. I hope it helps.