I think your problem can be a power surge.
Do you have an UPS?
If YES, Try to change it by another.
If NO, try with one!
We already changs the UPS for a new one, it is a 2HU Cabinet with redundancy power supply, also the windows 2000 server is a same machine. If it was a power supply problem, i think the server is going down but do not hang!
I had several "server-dies" with a unstable win98-client.
first i thought, a hardware-problem with SME.
unknown lockup at different times.
checked the memory with charlie's tool.
changed the NICs, removed the SCSI-controller (only for tape backup), removed all unneeded hardware, disabled all not needed bios-settings.
nothing helped.
then i checked the logfiles.
everytime when the win98-client runs about one hour i had a lockup at the server.
checked the win98-client and found an old installation of WinMX (music-exchange-client).
There are 18 Windows XP Workstations en about 12 Windows 98 workstations, no apllication WinMX is running.
Remember that the server hangs at evenings when al the client are offline and not during the day when all the client are online