Just get a new phone.
I have a W910i, and I think it's a piece of junk.
It has more problems than just shutting off randomly....
I will never buy another SE phone because of the drama I have had with this one.
If you do a bit of research on google, you will see you aren't alone with your W910i problems. SE has done NOTHING to remedy these issues. Their update service is poor too. Go with a more reputable brand... I know I will in future.
Andy
July 2008