Have you tried pointing the dev channel to an invalid URL (or even better, disconnecting from the network) on startup to see if you see the same behavior? That would give you a better idea of what the cause might be.
You could also call a secondary xml feed if the first one returns as invalid. It seems doubtful that it's the channel if it's working most of the time, but maybe you want to pop up a can't connect at this time message.
Sure. I'm stuck with just my phone for the time being but I can post a simple function when I get back unless someone beats me to it. Basically it just uses asyncgettostring to get the response code. If the response is not 200 then move on to the secondary feed. You could also pop up a one line dialog message if you don't get a 200 response. That way the user can tell you how often they see the warning message. If they see it frequently but are able to connect to the second feed then you can assume there's some connectivity issue between them and your server. Be sure to put your secondary feed and images on a server with a different domain. In fact you could just use dropbox as your backup for testing purposes.