Acer laptop Shuts Down after seconds

[Valionis]

My Acer Aspire 7520 shuts down, loses power 5 - 25 seconds after powering on. 
It started when I was playing a video game and the laptop suddenly shut down. I thought it was overheating, so I let it cool down. About an hour later I turned it on and it boot normally, but 45 min. later the laptop shut down again. Next time I tried, it turned off 20, 5 minutes, then 30, 20 seconds later. In those 20 s after pressing the power button no errors or beeps occur, just the Acer logo screen. I even get past Acer logo sometimes or go to Bios password prompt if i press F2, just before laptop powers off. This all happened in the course of a few hours, laptop was working fine beforehand, no overheating (think so cause games lagged when laptop was overheating previously). If I turn it on again, just after it powered off, the laptop shuts down in 7 seconds, as opposed to 25 seconds if i leave it alone for a while. Also, according to task manager CPU and RAM usage is normal when shut down happens.

Tried
Cleaning out the fan and the heat sink (wasn't that much dust).
Applying new silver thermal grease to CPU and GPU after removing the old one.
Powering on with only one RAM stick, then only the other.
Powering on without the battery, only on AC power. Then only with the battery in.
Powering on while putting pressure to the middle of the keyboard (read that this might pop the CMOS battery to it's slot if that's the problem).
Pressing the power button for 1 min.

There are a lot of people with Acer Aspire 7520 who have powering on and off in endless loop problem. Supposedly GPU reflow helps. I don't think my problem is the same, though because my laptop shuts down and doesn't restart. Also, my screen is working.
This is the only computer I have. Any help or tips would be greatly appreciated.

[Geek-9pm]

Do you know how to remove the Hard Drive?
Can you get a find a 'kive' Linux CD that will run o  your laptop?
Or, just the Microsoft memory test CD.

Here is the plan:
Carefully remove the HDD from the laptop.
Power on with the test CD in the drive. If needed, you may have to set the option in the BIOS.

See how long the test will run. Or, if Linux, play a simple game for haft an hour.

The idea is to see if it was really the hard drive that was overheating. That is why you do it without the hard drive. If it runs OK without the hard drive, then it must be the hard drive.

[Valionis]

I removed the hard drive, but laptop still shut down after a few seconds, even if i tried to enter bios.

[jason2074]

Remove other removable components. All RAM module(would beep normally) then power on. Wifi card then power on. Check the ventillation if air is coming to know if the fan is working before it shutsdown again. Any changes or improvement? Most likely the built-in chip for the video is fried so there is really less you could do at this point.

[Valionis]

No change after removal of Wifi card. Black screen (no Acer logo), no beeping, but laptop stays maybe 2 or 3 s longer than normal after removing both RAM modules. Fan is working. How can I determine if it's really my GPU that's at fault? Then I could try blasting it with a heat gun.

[jason2074]

Have you tried using AC adapter, no RAM modules and network card removed at the same time? Have you check that your CMOS battery is not drained or replacing it? The CPU might be your next step if all fails, while I've already stated the onboard chipset graphics might be dead or faulty.

[hthompson]

If you face the laptop, lift it up and on the left hand side towards the battery there is a vent (not the large vent that stretches across the front). This is where I tapped it and it worked. Have to say though I needed to give it more than just a gentle tap before it started working. Nevertheless I am amazed that this worked. I though it was going to be off to the repair workshop for a few weeks.

You can also try this.
http://www.techyv.com/questions/overheating-problem-acer-aspire-5050-notebook