Office XP problem with Dell Optiplex PCs

I am having problems with Winword 2002 on Optiplex GX 240 @ 1.8 Ghz. If you open Winword under normal circumstances and perform the everyday editing, modifying, deleting, printing functions the system will eventually lock up. This lockup usually occurs about 25 - 30 minutes into using the system. The system just looses all mouse/keyboard functionality and eventually may or may not automatically reboot.
I have Office XP installed locally on 2 dozen PCs all of which are Dell Optiplex PCs of different vintages. Each PC comes from the same OS setup, and all of the software is deployed using a Microsoft Installer or a Netware Zenworks package, so I can be assured that each machine is exactly the same except for the user settings, the audio/video/network drivers, and the computer name. The 3 machines that are experiencing this problem all have one common thread that is different that the other 20 + PCs, they are all Optiplex GX 240's running at 1.8 Ghz.