iMac Mess, I have no clue what's wrong.

Try the following first:
Reset SMC: http://support.apple.com/kb/HT1543
Reset PRAM and NVRAM: http://support.apple.com/kb/HT1379

Good grief! If you paid for the Apple Care, I would highly recommend using it as long as you can. They will migrate everything for you and you'll be back up and (hopefully) happy. You should have your original Windoz disk with the key on it and that is all the license you need.

I will review the other aspects of the problem and see if I can figure out what may be going wrong. I'm just an experienced end user and not an Apple Genius, but I usually blunder through. 🙂

I reviewed your write up of the problem and judging from that, the Apple Genius was probably on the right track thinking it's a hardware problem. From what I've read in the technical notes from Apple, modern iMac's reset their power management system by unplugging, waiting a few minutes or, at least 15 seconds (for the capacitors to discharge), and then power back up. You've done that and still have the problem. If it is hardware it would be the battery or the power supply, both of which are royal pains to change, but luckily are covered by the Apple Care.

Aside from that, you can use your Mac until it shuts down and note carefully the time. When you power back up, go into your console in the Applications/utilities folder and look to see what your system log was recording at shutdown. If there are loads of text on the screen, use the search window at the top of the console screen and search for "error". Each of lines of output are time stamped and will help you narrow down your area of concern. Take peeks at the other logs also around that time and see if they may have captured an event that would crash the OS. If you see something suspicious, write the exact wording down so that you can report that either here on the forum or to the Apple Genius doing your repair. Don't be afraid to report it, no one will think you're being silly if the message doesn't pertain to the problem.

BTW, if you are concerned about the failure so soon in the life of your Mac, don't worry. I'm a manufacturing engineer and most failures occur early in the life of any product, electronic or otherwise.

Believe it or not, when this problem is solved you'll look back on it as an enjoyable romp into the world of problem solving, so happy hunting. 😉

Ok I've gotta ask... Have you tried different AC plug and maybe take it to a friends to test on their power?

Or have you actually taken it to the Apple store and they tested it with their in house power?

AriStarHD,

Same problem here. Please come back to this thread and let us know how you make out.

My model:
- iMac (20" screen Mid-2007)
- OS 10.5.8

possibly heat related. run a few HD quicktime trailers simultaneously and monitor your cpu and gpu temps. loop the trailers so they play continuously for a few minutes and see if the imac shuts down, or the gui freezes, or you notice any other graphical glitches/anomolies. if so, you've got a damaged/defective logic board and/or video card.

Have you tried smelling salts?

Dave

Is the Firmware updated? Check the System Profiler's "Hardware Overview" to verify this. After the firmware is successfully applied to your Mac, your Boot ROM Version will be:
 IM71.007A.B03

If needed:
http://support.apple.com/downloads/iMacEFI_Firmware_Update_13

Check the plug. I had a friend who had a similar problem and it was just the plug falling out. Make sure it's securely plugged in and maybe tape it to make sure it doesn't fall out and see if the problem goes away.

Rob