Anyone else had trouble option booting with a 2010 Mac Pro? Can't get to startup manager. All I get is a blank screen. Apple says it must be a hardware problem after having me test everything else. Keyboard checks out. Can boot from install disk holding C key, but cannot change startup disk with prefs to install dvd or to a cloned drive. Have reinstalled SL 10.6.4 from install disk. Zapped pram and unplugged power, held power button 5-10 secs. and plugged in again - to reset computer according to apple tech. Thoughts?