It might be a heat issue. I've had 2 replacement handsets that kept resetting themselves due to heat issues. Once the phone gets damaged once due to heat (and this is a common issue with the Xperia), then the damage has been done.
My first handset reset itself when I was using Google Navigation while the phone was charging in the car, and that was how I proved it to the people in the shop: I plugged it in and messed around with maps for a minute and then it reset. Did it again, same thing.
It's worth mentioning that I managed to get a replacement through my operator in the UK *after* the 1 year manufacturer warranty passed because the phone was basically faulty.
