Domain access problems.

I have a network with 9 servers and about 30 users using DHCP and we’re having constant issues regarding which machines and servers are not only visible but accessable on the network. Some machines ‘see’ more than others in explorer and even when you can see a server (for example) it’s impossible to connect to it even using the administrator logon and password – a message is returned (at times) saying the network path is not available. Added to this, there are vast performance differences between users – for no apparent reason. I did a test on my laptop by de-installing and reinstalling TCP/IP but it had little effect. Any suggestions?