Is anybody else seeing an issue with Sierra and Late 2009 iMacs? With mine, the machine will freeze up but iTunes will keep playing and the mouse continues to move, However no mouse clicks are registered so can't drag, select menu items, move windows, etc. The one thing that seems to be consistent in all freezes is that Safari is running. The only way I know to gain control of the machine is a hard shutdown (hold down power button). SSH-ing into the machine shows that Windowserver is stuck. Anybody seen this or have any suggestions on how to resolve?