Title says it all. Every once in a while, my mid-2010 15" i5 MBP sits at the greyish Apple logo screen, after the "bong," with the wheel spinning forever. If I shut down and reboot, I'm at a log-on screen in about 15 seconds (which is my typical boot time, as I have an SSD). This is my first Mac after a long time on PCs, so I've got no idea where to start. All the posts I've found by searching pertain to earlier model machines using HDDs where the user can't get a boot at all; I'm only seeing this weirdness periodically. Can anybody offer some suggestions?