I can't be of any help I'm afraid, but I'm experiencing very similar problems to you. I've had my 770 for just over a week, and the connection i'm getting seems to be growing increasingly unstable. I managed to stay connected for about ten minutes at a time when I first got it, but now (and one firmware upgrade later, which may or may not have made the problem worse; currently on 45-8) i'm lucky if I can manage to maintain a connection for half a minute before it drops. Even with internet radio, it manages a few minutes at most before losing the connection.

I'm using an Edimax wireless router with MAC filtering, currently set to mixed mode, but I've tried B and G modes, and played around with lots of other settings, with no improvement. I'm pretty sure it's not a problem with my network hardware, given that none of the desktops or laptops in the house have ever had issues maintaining a connection.

I really hope there's a solution to this cause at the moment my 770 is virtually useless..