Someone said :
I also have an XperiaU. Most of the time it's fine but about once a month it seems to get a nervous breakdown and starts spontaneously shutting itself down (repeatedly when I restart it). Sometimes it even tells me that my PIN is wrong when it is not. I have to turn it off and take out the battery and then after a while I restart it again and it works ok for another month. It is a nuisance when it starts shutting down when I'm engaged in exchanging messages or calls.
The U's seem to have some problems random defects, alright. Mine would take a couple of days to recharge if I let the battery run out completely, though this only happened after I lost the original charger and bought a non-Sony brand replacement.
But I just learned to live with it.
Anyway, my mobile got robbed a couple of weeks ago so now I don't have this problem anymore.