If you have all diverts cancelled, this could be a resource issue. The Ace was never particularly well-endowed in that area. In the first instance, if you haven't already, put as much content (photos, music) onto an external sd card and move any apps as you can there as well (if your version of Android supports that). You could also look at your apps and delete any you don't use much.
If that doesn't improve the situation, try a factory reset - don't forget to back up your content first and make sure your contacts are fully sync'd. If you have an external sd card, I'd remove that first to make sure it doesn't get wiped in the process. Then run the phone vanilla, with no additional apps or content. If it's fine, put things back progressively and check your memory/storage as you go (Settings|Storage).
