Are you thinking that there is a routing loop, or a layer 2 loop ?
For both you really need a good diagram of the network to help you (it is a good thing to have anyway). It must show ALL items that are in the network and all the connections between them.
For a routing loop, you need to understand how the traffic is expected to flow, and try using the traceroute command to see if the packets visit the same router more than once.
If you think there is a layer 2 loop, then turn on spanning-tree on the switches, and see if the traffic levels drop, after a few minutes, and monitor for a while to see if this is now fixed. The diagram should help you detect where the loop is located.
A packet sniffer, such as WireShark can also help you to identify if the loop. Look for duplicate packets, with increased TTL. Look in several places around the network. If possible get a 10/100Mbps hub, and put it in-line with the network, and connect the PC with WireShark to the hub (only 3 conections to the hub), which allows you to easily see the traffic, with no config changes to the network.
There is no ‘magic bullet’ to fix a loop. It needs some investigation, and a good knowledge of the network topology. I hope these tips help to get you started.
You may like to provide a bit more information here, then it may be easier to give more directed assistance.