27 iMac keeps rebooting @ random

[Icedg]

I have a 2011 27 in iMac that I just acquired. Everything seems fine except for no reason it will decide to reboot.
Any ideas??

 

[old-wiz]

You need to post the console log of the crash.

 

[Icedg]

After two weeks of trouble free. It started doing it again. This was the last command on Console before it rebooted.

AVF encoder error: fail to create accelerator instance