I now know that my CPU is way too hot, and that hibernating only occurs when I hear the fans working hard. My CPU is at 80 - 90 °C at normal usage, that isn't normal, uh? I did some googling, and found this:
Great news! (Not the overheating... )
So now, you need to do some spring cleaning! Check your fans and vents for dust. (You might need to dissemble some parts to get in there. Be very careful when dissembling a laptop. Most manufactures have illustrated guides on this. If you're not careful, when you reassemble, you will have extra screws! Also, it's not like screws from desktop computers, where you can mix and match... every screw MUST go back to where it came from!! (Yes, in this case, size DOES matter! )
Hopefully, it's just a bad fan. There are ways to fix fans (oil the pin) but they are so cheap, just get a new one if that's the problem.