I have a Mac Pro running El Capitan 10.11.2 and Safari 9.0.2. I am experiencing repeated crashes where either screen goes dark and freezes or there is wild flashing that will not stop. When this happens, only alternative seems to be power off and reboot...HOWEVER in most instances reboot process seems to be clobbered..may take NUMEROUS attempts at "option" backup reboot or "command R" to get system back on line...OS has been reinstalled numerous times with NO positive results...HAS ANYONE ELSE EXPERIENCED THIS PROBLEM???...IS THERE A FIX???