Not sure if it's the same issue but I seem to have a working solution to my problem that was similar:
I experienced exactly the same problems as people have described here. Phone would randomly auto shutoff (and in some cases restart). I thought it may be related to network access or file IO as the restarts seemed more often when using the phone more heavily or when in transit and moving across areas of low network/signal quality (interestingly at work where we have a very strong signal it restarted less frequently). I had reset the phone and removed and reinstalled all apps as suggested by Sony. Waste of time. Kept happening. I was reluctantly about to send the phone to Sony although with little hope of them being able to resolve it based on comments here when it struck me that the behavior was very similar to how the phone shuts down when the battery is drained. As far as I could tell my battery was good. It hadn't suffered any of the problems that people have reported elsewherewith incorrect charged status and seemed to charge and discharge linearly but figured that I would see if a new battery solved the problem. It did. I replaced the battery this time last week and no restarts at all since. Previously it would have gone 10 to 20 times per day (the ones I would know about as there were probably more when I wasn't watching).
I can't guarantee this but in my situation it worked for me.
Word of warning: I bought a replacement battery on eBay. Read the reviews carefully. The first one I bought just didn't even charge! however the second one from a different seller works an absolute treat.
Hope this helps others. I was very close to hitting it with a hammer as it was so frustrating but now I have my Xperia V back and It's great again !!
If this does prove to be the solution and others can confirm we should be pushing Sony to ship replacement batteries as a matter of course to anyone who expriences this. @Samuel and the rest of the Sony Xperia Support Staff please can you comment on whether Sony will do this.
