Notebook freezes/restarts unexpectedly on heavy task/video, or when turned upside down.

I suspect having a recurring issue although my notebook was repaired under warranty in less than a year ago. It was repaired within 4 months after purchase due to the logic board/graphic card problem. And it was manufactured in mid 2012, but purchased in Jan 2014.

Recently, when I am running heavy tasks, such as video conferencing or video streaming with other documents running simultaneously, it shows either green, or colored squares over the screen (sometimes wiggling across the screen), and then freezes. Sometimes, it restarts unexpectedly.

Also, when I moved my notebook & turned it upside down to read the serial number underneath, it freezes as well.

Whenever the freezing occurs, I notice that the logic board or fan area is pretty hot. However, my fan is running normally. I did a hardware test, but found that there is no hardware problem. Everytime the freeze occurs, I could only force shutdown it with the power button. It runs normally when restarted, and only if the bottom part is not overheated.

From the discussions found online, it seems to be the graphic card problem which is wired near the logic board. If it were to be replaced, it would cost me quite a lot, but it seemed to solve the problem about a year ago. Yet, the problem might recur within a year or two. It bothers me with the uncertainty of when it would freeze while I am working. Does anyone have any idea that it is most likely the graphic card or logic board problem?