I had the same problem for few weeks since I have bought the phone (ICS 4.04 version).

I have found out that the cause for the problem in my case was the alarm clock!

So you need to freeze the android clockpackage.

I have used System Tuner application to do this (my phone is root) since freezing a system task requires the phone to be rooted

(I don't know if there is an application freezing system tasks without the need of the phone to be rooted)

Then I installed another clock alarm application from google-play for my usage.

I have verified that this is indeed the problem by unfreezing the clock task after a week without problems and then within a day the phone turned itself off. Again I have freezed the task and since then 2 weeks and I don't have the problem.

I hope it will be helpfull for you too.