We have a 2000 server with about 50 Windows 98 clients and a mixture of 10 and 10/100 network cards. This is in a school environment. New cabling CAT6 has been installed and we have installed new 3Com 5500 switches x 2. These use fibre optic links to other parts of the building not relating to this issue.
The switches are in the same room as the machines along with the server. What happens sometimes is that the clients do not get an IP from the server. When we run a winipcfg and click renew the address pops in. On one occasion it would not get an ip so I disconneted the cable from the switch for that machine and plugged it back in and it worked. This problem varies on 10 and 10/100 cards so I don't think the issue is there. The machines are a few years old but run well and are clean as a whistle.