As I said, it only happened a couple of times, maybe 3 at most, and in my opinion it was all due to the same thing (having about 50 tabs opened for months)Google Chrome caused the crash, according to the logs. Basically Google Chrome attempted to address a part of the graphic memory that was already being used, and the computer shut down.
You said it happened "a couple of times". How many times did it happen so far?
Also, Does it always happen with Chrome? If so, send a bug report to Google, the people who make Chrome. Does it happen with other programs? If there is, your graphic card might be failing. Have you tried using an external graphics card?