Your phone only has 512 MB of RAM, so that does put the system at risk for running low on RAM for open apps. I see a little broom icon at the upper left--do you have some kind of RAM booster/cleaner app running? Those apps can sometimes be counterproductive. I used to use All-In-One Toolbox until I noticed that whenever I ran it, it showed my RAM to be about 90% occupied, but when it wasn't running and I checked the RAM usage in the system menu, it was more like 70%.