Severe screen fragmentation

Question

Hi everyone,
I'm working on a 2.26GHz MacPro with 12GBs of RAM.  Notable installed software includes Adobe Master Collection and Final Cut Pro.
I have 3 (standard) Nvidia GeForce GT 120 graphics cards.  I also have three 27" cinema displays, one for each graphics card.
I'm running OSX 10.6.8 (snow leopard).
I am a video editor/graphic artist and spend a minimum of 8 hours on my computer a day.  More often than not, the computer works as a render engine overnight while I am away.  I have a tendancy to stream video while I'm working (on something that doesn't require my ears).  I manage to do a complete shut down about once a week.
About 8 months ago, while working and streaming video, the centre display went completely haywire.  Click the link to see a screen cap.
http://s1084.photobucket.com/albums/j413/apeszat/Mac%20problem/?action=view&curr ent=MACissue.jpg
Basically, the screen will (very suddenly) fragment and those fragments will get worse and move around if you try to move the mouse.  If you click and drag something from the affected screen onto one of the other screens, the problem will stay behind.  If you drag it back to the affected screen the problem will reappear.  The only way to stop the fragmentation and return the display to normal is to do an entire reboot of the system.  Nothing else will get rid of the fragmentation.  At this point I thought it was a graphics card issue since only the one screen was being affected.  A few weeks after this first issue, the problem starting happening on 2 out of 3 of the screens.  I had two certified Apple engineers come in to try and fix the problem.  Neither one could find any issues with the machine whatsoever.  All the hardware was tested.  All the software was tested and updated.  PRAM was flushed several times.  Nothing worked.  They both said they thought it was the display itself causing the issue since the issue didn't appear to travel between displays.  In fact, one of the engineers called Apple and Apple didn't not know what the problem was but confirmed that his theory was most likely accurate.
So two weeks ago I disconnected both of the displays that are causing issues.  I replaced one of the displays with a brand new one I had lying around (simply can't work with only one screen).  For a full week everything was fine.  The issue didn't come back.  So I had thought that the issue had been isolated and resolved. 
Until a few days ago.  Quite suddenly, my left-hand display fragmented one day while editing.  I did a reboot and it happened again within a few hours.  It has happened at least once a day on that screen ever since.  This morning the problem appeared on my right-hand screen (the brand new one I just plugged in).  Oddly enough, the issue has never happened on two screens simultaneously. 
Sometimes trying to tile the active applications will force the screen to behave, but most often it doesn't.  On a bad day, if I try to tile my apps, the screen experiencing the issue will simply not respond to the command.  The other windows will but everything on the affected screen does not tile.
I don't know what else to do or check.  I told the engineers that my feeling was that it might the logic board.  They adamantly told me that the logic board could not be causing these problems.  Nor the graphics cards.  Their rationale was that if it was a hardware issue it would appear all the time, not periodically.
The best they could come up with, besides the problem being the actual display, was that I'm getting electrical interference from a large HP plotter that I sit next to.
Has anyone else experienced a problem like this one?
I'd like to avoid just straight up buying another machine, but I'm at a complete loss.
Thanks!

Answer 1

I've run countless tests on the RAM itself.
That is not necessary on a Mac Pro tower, because it uses Error Correcting Code (ECC) memory, unless you have deliberately defeated that excellent feature by installing non-ECC RAM. The RAM stores syndrome bits (coded check bits) that can be used to detect and correct ALL single-bit errors on the fly. Double-bit Errors are detected and are intended to cause a kernel panic.
As long as ECC shows as enabled, and all memory shows as present at its expected size, and status of each module is OK, they are fine. Do not spend any more time looking at main RAM memory in your Mac Pro tower.
RE: your screenshot.
This shows no hardware failures likely to be detected by a diagnostic. The kinds of issues shown in your screenshot could be caused by software, but that is unlikely.
Debugging in general:
You have been through the easy solutions. Now you should substitute additional KNOWN GOOD items for potential problem items, BUT: you need to be very, very methodical, and only change one thing at a time.
For example, have you swapped display cards with each other? Have you tried a new card? Does the problem follow the card? The slot it is in? The display attached? The cables?