I upgraded to 4.3, without jailbreak. Phone seemed ok, but I turned it off and on again a few days later (normal power off). It started rebooting itself. I tried hard resets.
Eventually got to a stage where it needed the SIM unlock code but was otherwise working. Unlocking the SIM froze the device when the signal bars first showed (trying to register on the network, I guess). I tried resetting the network settings, no help.
I did a complete restore this morning, setup as new device, activated (officially unlocked). Still the same problem. Someone at work connected it to XCode on Mac, said they didn't really do anything, and it started working. I turned it off, on again, and it has the same problem. Any ideas?