It could be that in your settings you are set to not download MMS data, while roaming, or under some other condition. Sorry I don't remember them all. This would cause the download prompt. Being in a poor coverage area will sometimes also cause the download prompt, as the device cannot download enough data to satisfy the timeout failure requirements. MMS will not download over WiFi so that won't be any help to you if it is a weak signal issue. It is more likely that you are over burdening the app. The stock app is easily overcome by data if you have a few hundred MMS messages and a few thousand SMS messages. It runs out of resources trying to make those messages available instantly to you if you would want to look at them again. You could simply delete some of those messages until your app is working again, with the understanding that this will happen again or you could try a 3rd party app to take its place. As for which apps won't do this, almost any other app would handle it better. A quick Google will show list after list of peoples favorite apps for SMS. Each list will have differences but most of the good ones will mention these top 3, GoSMS, Handcent, Textra. As for which is actually best that will be up to you. Try a few. I know I've installed some for as short as a few minutes and some for as long as few weeks, before continuing the search. Currently I'm stuck on ZeroSMS... The only thing I don't like is there is a permenant ad at the bottom of my SMS list requesting that install their launcher. Sorry but I like Sense so that's not going to happen.
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