My 27" Imac (A1312/EMC2390) keeps shutting down unexpectedly with no warning or "screen of death". I replaced the power supply, reset the SMC, and zapped the PRAM, but the problem continues. Can anyone help me?

Do you still have AppleCare?

Was the power supply replaced under AppleCare or did you replace this yourself?

And are you sure this was the correct power supply for your model iMac?

Trying to elimate hardware issues.

Also, have you tried to disconnect your iMac completely from power for 5-10 mins and allow it to reset itself?

You can get a shutdown like that if the computer gets too hot. Make sure the air intakes on the bottom and the exhaust vent on the top are clean and are not blocked by something in your workspace like papers or the back of the computer being too close to part of the furniture. You need a good six inches (15cm) of clearance between the exhaust slot and other objects or surfaces.

Anti-virus/internet security software, especially the "pay for" kinds, can increase operating temps. So can third-party backup software furnished with many "name-brand" external hard drives.

Fan motors and/or temperature sensors can fail. If you have the original grey disks that shipped with your iMac, find the on that has Apple Hardware Test (AHT) and run it. If you have a failed heat sensor or fan motor, it will detect it with an error code containing "SNS" if it's a bad sensor or "MOT" if its a bad fan motor.

AHT instructions here:

http://support.apple.com/kb/HT1509

Also see if your iMac 27 falls under theis Apple Repair Extension Program:

iMac 1TB Seagate Hard Drive Replacement Program

Did you install OS X Snow Leopard from the original restore discs that came with the iMac?

Get iStat Pro widget, which will display temps and fan speeds. Post the temps in Celsius please, but only after it's been up for a while running with the programs that are open when the shutdowns occur.

http://www.macupdate.com/app/mac/20364/istat-pro

I Would do as Allan has suggested.

Some real,good advice there.

It's possible you might have missed something when you did the take down or reassembly of the iMac?

As experienced as I am working on Macs, the newer aluminum model iMacs are not designed to be easy to work on and upgrade/replace things and you need to have real patient and really pay attention to all of the little details, screws, clips and wired connections when working on these iMac models. It's too tedious and nerve wracking work that I won't ever try to undertake.

What are you showing for fan speeds?