I have the problem solved! On the main menu hit the menu button - Setting.Then click the "Sound and Display", scroll down to your display settings, and click Screen timeout and change it to "Never Timeout". After this setting you can delete long threads with no problem. The problem happens because while deleting a long thread (thus taking a long time) your phone (computer) wants to preform another action, such as the screen brightness changing, and that is what makes the phone freeze. After changing this setting just delete your messages with no problem. For me a 50 message txt took 30 seconds, and a 1000 message txt (with media) took 10 mins.