I searched a solution for this question along amny lines, but I still haven't got anything solid. Please help me here.

Things with Ubuntu were working absolutely fine, but suddenly, the laptop started overheating whenever I logged into Ubuntu. And that lead to automatic shutdown. I realised that it was definitely due to heat. This overheat crossed a limit and it started affecting even when I logged into Windows. With appropriate service, the laptop fan was cleaned and things are absolutely back to normal in Windows 8. But, there are still problems with using Ubuntu.

1. After a while, it closes automatically. And, if the laptop is on battery, it doesn't switch on immediately, unless you connect to power. I tried without battery and the problem is still the same. So , battery problems are ruled out.

2. My laptop is HP- pavilion, g6-2313ax notebook. I would like to stress again, it was working really fine, but one fine day, this problem started to occur. When searchd over net, they said something about the kernel version and it being updated could be a reason.

3. I tried the one suggested in this link as well, but it did not work!
http://www.webupd8.org/2011/06/linux...issue-fix.html.

4. I got a small intuition as to the birth of this problem. I reinstalled Windows 8 once with ubuntu on it and thus it removed the grub. To retrieve it, I followed few online steps!

http://howtoubuntu.org/how-to-repair...d#.UwlfH_mSx5I
.
Ever since the I got the grub back, I have been facing this issue. But I cleared my whole hard disk and properly installed Ubuntu and Windows again. Still, the problem exists.

Can you please tell me the possible error and the perfect solution for it? I have searched for really long and am new to ubuntu. So, I need your help, as geeky as possible. I have completely listed down all that I did and my case here. Please help. Thanks in advance

You need to take the laptop in for servicing.

At a minimum, the ventilation needs to be cleaned out, and possibly you have a fan that needs to be replaced.

Thank you for your suggestion. This problem persisted after doing both and even after applying cooling gel over the processors, during the service. Some said this could be due to not updating the bios. Any thoughts on that?

It shouldnt be anything to do with not updating the BIOS.

If it was woking fine before I'd guess hardware problem, but you seem to have covered that. So possibly some dodgy update caused the issue, did you get any updates just before this happened?

Its also possible that the video drivers have been changed somehow, that would explain it. Since thats a 'switachable graphics' AMD APU + AMD GPU system, and if you leave the GPU running it will use more power and create more heat.

Trying out random guides will not help you. That guide is for kernel 2.6.38, which was used with ubuntu 11.04. Ubuntu 13.04 uses kernel 3.8, and shouldnt have the power iusses that 2.6.38 did.

and guess what, 13.04 is 'end of life, out of support', so you might as well either go back to 12.04 (5 years support for ubuntu, various lenghts for x/l/kubuntu etc.). Or get 13.10 and repeat in a few months (its out of support in july 2014). Or get a different distro with decent support lenght.....

The overheating is most likely caused by the two AMD GPUs. Which driver (free radeon or proprietary fglrx) are you using, in which version?

Can you please tell me how I can see that?

Run the command and post the output here.