Well for what it's worth I'm experiencing the same issue with my 27 inch i7 quad core fusion drive imac.
I brought it to apple a couple months ago they said it was fine and I should erase and reinstall the system.
Of course that will take a lot of work to bring all my programs back on line so I've been avoiding like the plague. I did this years ago it took two days to reinstall things.
It is now happening many times a day. 10 shutdowns yesterday 6 the day before 5 the day before that 3 today.
Back your machine up if you can right now it appears to get worse.
Get the important things secure like keychain passwords, 1 passwords, contacts calendars, get them so you can re-enter them.
I fear I'm going to halve to go down this road of pain. reinstall bootcamp windows 7 ,Parallels, and a bunch of other software.
For what it's worth try running in "Safe Mode" (Start with "Shift button held until machine wants you to choose your user) .So far I have doubled my good run time today, but functions are different now.
Apple isn't real keen on helping is the impression I'm getting. They keep saying it's software. Many things I've read online about this issue say a software reinstall didn't help. However I'm also noticing wildly fluctuating startup times. Occasionally machine can't find the internet without me forcing it to with system control panel and unplug the ethernet cable. Today I had an issue with mail being "Online" with a warning symbol saying couldn't find server. Restart fixed that.
Help.
<Edited by Host>
