Wanna know what the issue is? Wifi sleep policy.
If you're using Wifi, after 15 min. the wifi goes dormant and disconnects obviously to save battery life. To disable this:
Menu > Settings > Wireless & Networks > Wi-Fi Settings > Menu > Advanced > Wi-Fi Sleep Policy > Never
That will keep the wifi connected at all times. Obviously if you turn wifi off and just use the mobile network, GTalks connection will be determined by the network rather than wifi, but if you're using Wifi, that's the reason it's disconnecting.