Keep getting the Yontoo.C virus on Dell Optiplex 7010 running Symantec Endpoint Protection Version 12.1.4013.4013 doesnt discover it until reboot

Keep getting the Yontoo.C virus on Dell Optiplex 7010 running Symantec Endpoint Protection Version 12.1.4013.4013 doesnt discover it until reboot. Symptoms: Once infected CPU usage jumps to almost 100% and stays there until the reboot. Why will Norton not catch and contain this until the reboot? I have updated definitions several times. It is annoying to have to reboot mid day since everything I am working on must be shut down and reopened which eats up 1-2 hrs of my day the last two infections. Please Help!!