Hello,
Open console in Applications>Utilities, check the system log for the date/time of the last problem for clues.
Open console in Applications>Utilities, check the system log for the date/time of the last problem & the Startup right after that for clues.
click on the top line we want, Shift plus click on the bottom line we want, CMD+c to copy, then paste the text in a reply with CMD+v
A few dozen lines should be about right, hundreds or thousands won't likely be readable. 🙂
Or...
One way to test is to Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, Test for problem in Safe Mode...
PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive
Reboot, test again.
If it only does it in Regular Boot, then it could be some hardware problem like Video card, (Quartz is turned off in Safe Mode), or Airport, or some USB or Firewire device, or 3rd party add-on, Check System Preferences>Accounts (Users & Groups in later OSX versions)>Login Items window to see if it or something relevant is listed.
Check the System Preferences>Other Row, for 3rd party Pref Panes.
Also look in these if they exist, some are invisible...
/private/var/run/StartupItems
/Library/StartupItems
/System/Library/StartupItems
/System/Library/LaunchDaemons
/Library/LaunchDaemons