This may be a stab in the dark, but how old is the internal backup battery? A weak battery can cause odd symptoms. In my G4, I had partial startups and then a shutdown. In my Mac Pro, there were problems waking from sleep. In both cases, replacing the battery cured it.
Apple says that the battery in iMacs is not user-replaceable so you may have to go to Apple to have it done.
Worth a thought.
EDIT: I seem to recall that I asked about the battery in a previous thread and that it wasn't an issue. If that was your thread, ignore this reply. If not, keep it in mind.
Last edited by jchuzi; 09/23/12 09:44 AM.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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