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