Network slows down over time

I have a Windows 2000 Domain Controller and had Windows 2000 and 98
workstations and everthing was fine.

I now have upgraded some and added some Windows 2000 Pro workstations and in
every case the network slows down over time. A reboot fixes the problem for
a short time but soon it starts slowing down again.

The slowdown shows up when I browse, copy files, option files, run programs
that access databases, etc...

I am convinced it's not a hardware problem since it happens with every
computer and I have tried various hubs and routers. Any idea what it could
be?