I have aspire 4736G and i have the same problem to. The main reason for this automated shutdown is because the heatsink problem . From my case, my fan is mess up with dust because Im always working my laptop on the bed. Maybe I can suggest some tips to recognize if your have some similar problem like mine.
1) Found out what your GPU/CPU temperature are
You can easily download this useful windows desktop gadget from this link.
After that you can made some experience with your laptop. The gadget will show the temperature of your GPU (mine is geforce G105M). Usually when u running the 3D program such as games, 3D software even google earth also the GPU unit temperature would be increase.
So what you can do is run any game( or any 3D software), but DO NOT RUN IN FULL SCREEN as our objective to observe what is the temperature show on this gadget. You can see the temperature of your GPU would increasing. If the temp achieve to 99-100C then your notebook would automatically shutdown. This because the BIOS is already program to force shutdown if the the temp is getting to 100C. (or sometime 105C) to prevent a absolute damage to your laptop.
2) fast Solution: Buy notebook cooling pad.
Buy the cooling pad and make sure, the fan of the cooling pad is directly to your heatsink ventilation. (It was at bottom left side of your laptop). Cooler Master X-lite or X2 series is good for the aspire 4736G model as the huge diameter fan of this cooler pad can cover up your heatsink ventilation. Try to observe your temp. again. If u dont have any notebook cooling pad, you can experimentally using normal fan. (stand fan should be good). Just face up the bottom of your laptop directly to the fan and u can see the temperature would be decrease.
3)difficult Solution: Clean up your heatsink.
This would require you to disamble your 4736G totally. I would not telling how to do the best way you need to send your laptop to the service center or technician. However if u really want to, u can download the service guide for 4736G at
Another u should know that, the heasink cover up the CPU and GPU unit together not like other laptop model which are separated. If you have like problem like just in stand by mode (just running window without any application) and suddenly shut down. That mean you are totally MESS UP with your heatsink. But then it could maybe from the other problem occur.