Help find what causes MBP reboot in 10.10.2

I have a Mac Book Pro from 2011. I've been updating it to all OSX versions since I bought it and recently I updated to Yosemite. At the time I also changed memory + HDD. I cloned my old drive to the new and swapped them.

Everything worked fine for 2 months but in the last 2 weeks I've had it crashing on me twice. It has never happened before.

Do you have any tips for how I can find what's causing the problem? I've been trying to look in the system.log file but I'm not sure what to look for. There is a lot of output there that doesn't feel quite right though...

Or should I just make my first re-install since I bought instead?

Thanks in advance for all help that can point me in the right direction.