My iPhone 4s closes applications randomly while I'm using them?

I have an iPhone 4s that I purchased less than a month ago. While I'm using applications such as instagram and games, the screen turns black and then brings me to the home page. Facebook is the only application that does not close randomly. Instagram closes the most frequently. I went to the Apple store in the mall and the "tech support" moron told me to uninstall everything and then re install it all. I did this while I was there, and it still didn't fix the problem. He then told me that I probably dropped it and broke it. I have not dropped it… (He didn't seem to know what he was talking about at all.) Does anyone know of any way this can be fixed? It is very annoying!

Your phone is slow, and it can't process all the apps that are on. Double click the home button and hold on the screen then delete all your apps that are on. I believe that this is a way of solving your problem.

I had this same problem with my iphone 3GS I had to delete a couple of apps there were taking up too much space then restart the phone. Make sure you apps are backed up I made that mistake. If that does not resolve the issue. Check out the link provided the guy knows what he is talking about and might be able to help you! Best of luck!