You're correct about MathLib, it used to be an extension but was merged into the system way back in 7.5 (actually I think 7.5.3).
I'm wondering if the problem might be that MathLib is not always being loading properly into the system's memory from the Mac OS ROM file in the System Folder.
You might try trashing the "Mac OS Preferences" from inside the Preferences Folder (within the System Folder) and restarting the computer(s). If that doesn't fix it, trash the "System Preferences." Of course, the user will have to reset a few preferences after trashing either or both of these preferences files.
Also, (I can't be sure of this but...) I don't know why a person couldn't replace the Mac OS ROM file itself, from the System Folder on the original System CD. If trashing the preferences didn't help, I'd sure give that a try. If you do this, just make sure you copy it from the right CD! Mismatching the Mac OS ROM version and the System version will definitely cause problems.
Hope this helps,