Sony Ericsson Xperia Arc randomly reboots
You need to update your phone to the latest firmware 4.0.2.A.0.62. I bet yours is 4.0.2.A.0.58

If you can't update the phone over OTA or PC Suite, use WOTAN to update the phone to 4.0.2.A.0.62. The reboot problem occurred on nearly all SE Xperia Models.