Network analyzer on the local segment. If it really is just on server it's going to be a bad cable segment, gabbering, excessive broadcasts or something of the sort. You need to be able to really see what's going on on that network. The best way (not the only, but in my experience the best) is with a network analyzer installed on the network segment itself. And you need someone who knows how to use it.
Really the easy way is to run MRTG Graphs on your switches. It will 9 times out of 10 tell you which port all the chatter is coming from. If you are talking about issues on a multi-segment network, OR you only have a single dedicated box on someone elses network you will want to put the ethernet card into permisc. mode and sniff for the booger causing trouble. In a Unix environment you have tcpdump (linux/freebsd) or snoop (solaris) on a Windows box or most unix systems you can also use Ethereal. a very nice sniffer