First issue is, (i think) the power button is broken, because of my 4 years old son oftenly push it so hard.
And then it begin to randomly shutdown, first thought is, maybe the power button is pushing a some kind of hardware inside so the device is react like it was being turn off.
Sometimes i can still turn on the device with the power button, but when it does, it came the second issue, the device fail to reach the bootvideo or reach it but then shutdown itself again and it began to restart again, and failed again.
But the strange is, i can power on the device without any fail if only the charger is mounted, and as long as the charger is mounted, it never get shutdown itself.
Any solution for this guys? or maybe some kind of battery or AC or whatever hardware is probably got some errors?
Or maybe it's just software issues? I don't mind to reflashing it, but just want to make sure it will solved the problems (too busy to restore all the files and apps)
And my N9 is in some kind of stock configuration, no N9QT installed.
Thanks for any answers.

When you manage to get it up with charger connected, will it shutdown immediately when you disconnect it?
What does "Settings->Device->Battery" say about your battery health?

nope, as i said before, it randomly shutdown
i give it try a several testing and observing some behaviours:
1. stay untouch all day, and it still on
2. just a click to power button, sometimes it's getting locked/wake up/still on, sometimes it turn off
3. when processing a heavy task, like opera mobile, several apps running in background, it turn off itself
4. light activity like side swiping, or click the notification bar, it turn off itself
I can't make sure what's wrong with this device.

And battery said: Good

Oh, i forgot something, i always get auto-booting as soon as i plug in the charger, which is strange, considering the normal behaviour when device is off and get charge is just get the battery icon in LPM and stays off.

How old is the battery?
Put ever to extreme conditions (temperature high/low).

I would try to get a new batt and exchange it, as it sounds a bit like voltage drop and device shutdown.

But it could also be faulty HW power button, hard to say...

i never change the battery yet, maybe like 2 years
same thought, its kinda like the hardware problem rather than software

Or maybe, my N9 getting jealous on me, because the problem begins when i start spend lot more time on my Lumia rather on my N9

Had a similar set of problems recently, and that definitely was the power button. Replacing it sorted the problem for me.

Did you repair it by yourself?
Have succesfully dismantle a Nokia N8, still no guts on N9

I didn't, though it doesn't look too bad to replace.