Dude I have no idea. Maybe try using gingerbread for a bit- go around root it. Try kernels and etc. Then use Sony's update service (on phone and I can't give you a link ,google it). Install official ics trough there and again root it(there is a post in ray development). Try putting 6.2.5 kernel and then ray flashtool package(tick no final verification). Then install ka18 play with it and reboot some times, boot stock kernel try it return back to 6.2.5( at all just test it). And flash kernel with flashtool as I said. And make a new administrator user in windows - reinstall flashtool and Sony pc companion and install flashtool drivers. Say in tread if your phone is ok or not
Running out of ideas mate :-(
Sent from my ST18i using XDA