notebook freezes after hibernation

Hi

I have an acer notebook with ubuntu 14.04 LTS that frequently freezes when it goes into hibeation mode. I lift up the screen, the power light is already on, I press the power button and there is no response. I also notice that the notebook is hot, as if it was in operation mode.

The only way to boot it is to remove the power cord and battery and then press the power button.

Here is a copy of the syslog at a time when this happened last time:
http://www.pastebin.ca/3603562

It seems that the system froze on May 18 23:46:13 and booted back on May 19 00:35:26.
Anybody can shed some light as to why it happened?