Just got a new imac 27" and it has been turning the sound on and off byitself like someone was hitting the mute Key on the keyboard over and over. It will work fine for a while and then it will just start flashing off and on over and over. I tried restarting, looking in the sound menu, etc but couldn't find anything. The speakers work fine and the noise is good when it is working but the constant on and off is an issue. Hoping a simple fix rather than return.