PC Lockups/Reboots

We have 6 Gateway E3400 PCs running Win 98, Office 2000 that have all started experiencing mysterious lockup problems. The lock ups seems to occur most often when switching between applications, when printing word documents or surfing the internet. I have even experienced 3 of these PC’s rebooting when trying to print, or surf the internet. Each PC is equipped with a 3COM NIC and is on a LAN. I have replaced the power supplies, reseated memory, replaced Hard Drives and video cards and reinstalled OP SYS and OFFICE downloaded all drivers and updates for hardware/software, and I still experience this problem. Any help would be appreciated.