I got a Macbook air 2011 bought sometime in July last year. Till date, I have experienced 5 screen froze and 1 dead issue. For the dead issue which happened today, the system basically refused to respond and the screen was blank. I called apple care and was advised to press shift+control+option+power buttons together. The system woke up after the power button was pressed. Need your advice. Although the system is working now, do I send it for servicing so that Apple could thoroughly check it?