My iMac has been perfect until about an hour ago, it is now freezing, sort of!!!! The mouse moves around but wont click on ANYTHING. The only way to do anything is to hold down the power button. I was using a mighty mouse but have plugged back in my USB one, not much better. After I power it back up it will work slowly for a bit, but then stops again. Its about 18 months old and all updates are done.???? any suggestions ?