I don't know much about servers and knows a little about cisco. We have a server in the campus data center and we have some users in a building in the same campus having slowness problems connectingto the server in the afternoons. We have other users in a seperate building and different vlan who can connect to the server with no problems. How can start looking into this problem and why would be
some reasons for this to happen?
Not knowing your network setup you need to look at all the connections between the slow vlan and your server for errors , speed/duplex mismatches and somehow get a look at the utilization of those connecting links .
I didn't think it was a duplex problem because you said it is only in the afternoon. This is why I suggested that you get a sniffing tool and see what traffic is taking up all of the bandwidth.
check if there are any backups going at that time. make sure that no one is using Norton Ghost or similar application, that floods the network.