I took my iPhone 4 to the Apple store based on this problem. The iPhone crashes during calls, when spawning new apps, when navigating an existing app. They suggested it was due to too many concurrent Apps running. Since that time I have sought to keep the number of apps down, but continue to experience crashes. Any suggestions?
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.