IMAC 27 inch late 2013 spontaneously restarting after recent download of Safari 9.0.3 and security update.

IMAC now decides to restart on its own. Can be working on a document, online or just sitting idle and the unit will decide on restarting.

Never had this problem since I got the computer. Was something wrong with the Safari and recent security update?

I have run disk permissions over and over.

Running Yosemite 10.5.5

Any help appreciated.