IMacs deployed for Staff use randomly freezing

Question

This school year we deployed over 300 new iMacs. Like in years past all teacher user accounts come from OpenLDAP. All of their files are kept on a Mac File server. Computer don't have bluetooth (edu model), Airport is disabled, with all machines hardwired over 100Mb Copper or 1Gb Copper. From the beginning of the year until the end of November we had no complaints reported. At the end of November I enabled through WGM Screen savers for when the users are logged in and idle. As a "Often" preference I have set the flurry screen saver to come on after the machine going idle for 600 seconds. This setting was on all last year for 10.5 and we had no issues. How it got disabled from last year to this year I don't know. Along with 600 second (10 minutes) screen savers we also employ:
*90 minute in-activity logout
*1 hour display sleep
**Computers are not set to sleep. Computer Hard Disks are also not set to sleep.**
Since making this change at the end of November this is pretty much what we are experiencing now:
"Teacher computers appear to be freezing or unresponsive after coming out of screen saver AND the machine has set idle for a long period of time. Screen saver alone does not cause the problem. Sitting idle does. At 90 minutes teacher machines attempt to gracefully log off their users. This log off attempts to close all open programs. Programs that require user intervention to quit (I.E. unsaved files) will halt the log off process. Once the process gets halted the machine acts like something else has its focus. You can't left click with the mouse on anything. A random application name (which is open) will appear beside the Apple Logo in the top left hand corner regardless of what you click."
Force quitting the finder doesn't do anything. Forcing a log out doesn't work either because whatever halted the log off to begin with is still open and you can't select it to close it. At this point all the end user can do is shut off the machine and then boot back up. Once booted up again everything is fine.
Crazy thing about this. Out of almost 300 Mac using teachers only about 20 people have reported this problem. I have 5 buildings which are Mac Land... the 20 people with the problem are spread out randomly over those 5 buildings.
Things we have tried:
*Running down the whole "open program thing" to see if its one program in particular. As far as I can tell its not.
*10.6.6 update on all machines and servers
*Swapping out users machine with a known working machine
*Moving user's home directory to a different server with far less traffic.
*Replacing keyboard and mouse.
*Zap PRAM
*Check/Repair HD permissions
*Reset all caches (Boot into safe mode then reboot.)
*Change out network cables on the machine
*Change port which computer is plugged into on our switch gear.
*Reset SMU
*Wipe out all of Users Library folder and let preferences and what not create again on their own.
Most frustrating about this problem is a staff member will call with the issue. I'll connect to their machine look it over. Theres nothing weird in the logs on the machine. Can't find anything wrong. I'll do one of the aforementioned things and then I won't hear from them for a week or so, but just as soon as you think its gone its back. Near as I can tell this doesn't happen to students. Weather thats because they don't use the machines the same way the teachers do or something else is the key I don't know.
I'm looking for suggestions at this point. I've tried everything I can think of and have had no luck working this out. I was holding out for the 10.6.6 update hoping this would just go away as it was rolled out to the end users, but thats not working either.
Any suggestions anyone can give would be greatly appreciated... Thanks for reading.

Answer 1

No airport/wireless. We disable Airport through WGM and also remove it from our district images so that after format unless your an admin and you go to System->Network the airport isn't even setup.
Staff does use USB keys, they have the right to do so through WGM, so at times they can carry their files around. Most of the time they are not plugged in but I will look around and double check.