After thinking that the worst might be happening with my n900, ~phoenix~ opened my eyes to see a more simple problem, MY BATTERY WAS MOVING , this was the cause of my phone turns off randomly.

It was not too obvious find the problem, beacuse removing the back cover and move the battery did not turn off the phone; but the last time my phone passed out was sliding the screen up when it reached the superior limit ( when a little click takes place ); that make me think that was probably the battery .

I bent these two support points of the battery ( see next picture ) and problem solved



And that would be a design problem ,thanks to all for your replies.

best regards