I am having a near daily crash on my iPhone 4. Has occurred with every iOS 5.x release
Your first step for troubleshooting should be to restore the phone. I'm surprised that they didn't tell you that at the Apple store. It might help you to set it up as a new phone instead of as a back up when you get the option. You would have to reinstall all your apps and reset all of your settings but it's the cleanest start.