I also think it is a RAM issue, if memtest checks out okay, try re-seating your memory into the other two slots. If that still doesn't help... I'm curious how your idle temps are 10-11c, do you not have heat on? Your computer should never be cooler than the ambient temperature of the room. If your room is say 21c then your computer should at least be 21c on idle, if this is not the case maybe your CPU is not registering the right temperatures and is in fact overheating. How does it shut down? Just straight off, or Blue Screen?