Sometimes, when I turn on my iMac (10.6.2), the system will not load, you can see the grey screen but empty, no apple logo there. I have to force a shutdown pressing the power button and when I turn it on for the second time it will work properly. It only happens once in a while but I cannot understand why, since nothing changes. Any ideas? Thanks. PS.- I have verified my disk with the disk utility and everything seems fine.