It's been happening for a while now and I had hoped 10.5.3 would fix. When I choose to restart or shutdown, 90% of the time it hangs on grey screen after logging out and I have to hold down power button to shutdown. No apps hanging. Also have problem where when I open the cover to bring my MBA out of standby mode, I'll be presented with the logon screen. I'll type my password and my screen goes blank like it's gone back to sleep but the power light is fully lit. Have to close cover, wait till it goes on standby, then open cover to bring up password logon again. Should I reinstall OS X again?