Twice in the past 3 weeks my imac would not start per the usual way A. press the space bar since it's asleep. B. push the power button since it off. Both times I had to hold the power button for 8 seconds, wait a few more seconds then press the power button=> booted up fine. In each case I don't remember if I had previously shut it off or put it to sleep. Ideas? Do I need to be worried? Works like a champ in every other way.