My iMac was rock stable, right up until I installed Mountain Lion on it. Then, at random times, it would suddenly reboot. Fortunately, most of it seemed to happen at night – I’d get up in the morning and find my Mac had rebooted.
So, I finally got around to investigating the problem – I had’t seen any good blog posts on it (most of them were things like take it to a Mac Genius). Finally, I found a hint buried in an Apple forum: the problem stems from Spotlight. The indexes for Spotlight get corrupted, and that causes the reboot. Ah-ha! So the solution is pretty simple – have Spotlight dump all it’s current stuff, and build up again from new. Of course, there’s not an obviously marked way to do this 🙂
Go into System Preference, and select Spotlight. Click on the Privacy tab. There probably isn’t anything listed under “Prevent Spotlight from searching these locations:” We’re going to add one. Hit the “+” at the bottom, and select your Mac’s hard drive, and click Choose. It’s now in the list.
Close System Preferences, and reboot your Mac once. Then, go back into System Preferences -> Spotlight -> Privacy, and remove the drive you added. Just click on the drive in the list, then click “-” at the bottom of the list.
After that, everything had been going great! 🙂 No more random reboots for me 🙂
One additional bit: Having Mountain Lion randomly reboot is a lot less painful than when versions older than Lion would reboot. Most of the apps I use are state aware, so for the most part when the iMac comes back up restores all my workspaces to what it was before 🙂