my GF had the exact same fone with the exact same symptoms.
she didnt have warrenty either, but the fone was covered anyway. even if sony didnt cover it, all electrical goods have to have atleast a 12month warrenty anyway
anyway she took it back to the shop, they updated firmware. few days later - same problems. they tried reinstalling the update, again a few days later it was crashing again. 3rd time the shop sent it back to sony, who just replaced it with a newer version of the fone free of charge. she says the newer version licks balls tho - prefered the old one.