Sudden Shutdown of iMacs with 16GB of RAM

Question

An ongoing discussion of iMac shutdowns over at Apple Discussion has prompted me to try to get some answers to this issue that we too are seeing in one of our i5 iMacs with 16GB of 3rd Party RAM.
http://discussions.apple.com/messageview.jspa?messageID=13311466&stqc=true
Recently we found that the issue could be repeated when transforming a layer in Photoshop CS5. Dragging the image around, resizing it a few times will consistently cause a sudden shutdown of the machine. Not a standard shutdown or kernel panic, but an instant off to black screen. The machine will boot right back up with no problem.
Further, we also found that this would occur when the Photoshop tile cache size was set to 128k. Setting it up to 1024k seemed to get rid of the issue and we were never able to recreate the issue when set to 1024k tile cache.
Now I would think to blame Photoshop, but it happened in CS4 and CS5. It also has happened in other apps as well. It just so happens that Photoshop
is doing something that exacerbates the issue and triggers it.
Now, my question is this. And it is rather technical in nature, so if any Adobe techs are reading this, I would love your take on it.
I'd like to know what is going on in terms of data exchange between, RAM, video card or swap space when an image is being transformed.
Does the change in the tile cache size change how large the chunks of data are that are cached to the video card as well?
I'm wondering what the change in tile cache size could possibly be changing that could alleviate this issue.
There are also a number of reports that say removing 4 or 8 GB to leave 12GB or 8GB configs alleviates this issue as well. Do machines with smaller RAM configs exchange data in different size blocks?
I hope this all makes sense. Any info would be appreciated.
Scott

Answer 1

Not a standard shutdown or kernel panic, but an instant off to black screen.
That means that you have a hardware problem, probably in the RAM, but possibly in the motherboard.  (having inadequate power supply to the RAM is a common problem that causes this - could be a bad capacitor, or a bad design)
An Apple technician should be able to put a scope on the power and signal pins of the RAM and see the problem happen (looking for a capacitive discharge pattern in the signals, dropping below signal threshold).
Changing the tile and cache size just changes the RAM access pattern slightly, but doesn't affect GPU usage much.
Yes, Photoshop moves a lot of data in and out of RAM with highly optimized code - which tends to expose problems with the hardware sooner than other applications.