I have a 24 iMac with a 2.8GHz Intel Core 2 Duo. Recently when I turned on the iMac the only thing that came up was a blank white screen. Now, I have to turn the iMac on several times in order for a proper screen to come up . How do I correct this problem? My AppleCare has expired. * Last night I posted a similar message, but I did not see the posting this morning.