Test Messages being erased unexpectedly iPhone 3G

After upgrading to the 4.0 OS, my phone has deleted my text history twice now. What happens is that I get a text a message from someone and click view and then the iphone will freeze for a few seconds and go to the lock screen. I then slide to unlock and go to my texts and they are all gone. Im not sure what is happening. Please help! I know how to restore my text messages. I just want to know why this is happening. Thanks for any help you can give me!
iOS 4