Hi, I am experiencing a problem with my iMac. The iMac works fine however if I shutdown the computer and 15 minutes I try to turn it on again it shows the white mac logo and then the display goes off. If I use a external display I can use the iMac but the iMac display does work just the external display. If turn the iMac off and turn it back on let´s say 3 or 4 hours later the iMac display works just fine... It seems that I have to wait around 3 or 4 hours to use the iMac display if I turn it off and turn it bak on. Please help.. If noone have the answer for this strange behavior please let me know where I can get an answer. I forgot to mention that a couple of months after I bought the iMac the display was changed with the warranty due of there were issues with some pixels. Now the warranty has expired.