Desktop covered in multi-coloured, pixilated pattern

Question

I've read other threads where people seem to be having the same problem I'm seeing, and the screen shots look identical. I am not convinced it's a hardware issue in my case, and none of the solutions I've seen recommended resolve the problem. Due to reasons which become apparent in the description below, I'm convinced the problem is NOT hardware related.
The Environment:
I'm maintaining 3 college computer labs (well, 3 Mac labs, I won't mention the PC's); I manage the client side, a co-worker manages the servers; The Mac labs currently hold approximately 100 iMacs, 2 different models, both models are intel, but two model years apart; ALL clients are running 10.5.5, HOWEVER, the servers are running sever 10.4.?; All authentication is done through Active Directory on a Windows Server 2003 based, multiple-domain network; student home directories are network-based on 2 xserves; Approximately 300+ Mac-based student users; We are using AFP, not SMB;
The Issue:
Aside from the expected (constant) issue of corrupted preferences, I've had two students experience an odd graphic problem: Seconds after logging in, the entire desktop region of the GUI is covered in a pixelated, muli-colored pattern. Sometimes the aliases will be uncovered, but usually are. Usually the dock remains free of the pattern, not always. Usually the menubar is mostly covered by the pattern, not always. It is still possible to interact with aliases and menu selections, if you know where to click, and the area immediately around the interface item 'clears'. Grabbing and dragging a window around the desktop will (for the current session) clear the pattern. If you click on the Apple icon in the menu bar, and drag to the right, the pattern is cleared of the pattern. If the user logs out, their next session repeats the problem of having a pattern cover everything.
The problem exists for those users alone and follows them from computer to computer, regardless of which model they are logging in on. Other students can log in on a specific computer immediately before, and immediately after, but only those two specific students would get the pattern over the desktop. The problem seemed to go away for ONE student, as mysteriously as it started. This second student just started having this "frost effect" today. (I don't know how else to describe it, it's as though the desktop was covered in multi-coloured, electronic frost which scraps away when you drag an object around the desktop).
With both students, I've tried deleting their saved preferences and cached info from their libraries, but this has no effect, the "frost" returns at their next log in.
This is more of an annoyance than a liability, but I would like to be able to resolve this problem for the student if I can, and for any future students that run into this same issue.
Any advice or suggestions would be most appreciated.

Answer 1

I must not have found that thread (it's hard to find accurate search results for "weird pattern"), I'll check it out, thank you.
BUT... I can't update the OS version for the images in mid-semester. We've had other issues that we specifically addressed with 10.5.5 over the winter break. Without extensive testing of 10.5.6, my manager would serve me on a platter to the Board of Governors. If it was fixed in a version update, I'm stuck with it until May. Thank you for your reply though.