You should plug directly into the core switch and run a command to view the port utilisation and cpu utilisation. If any ports are running high, (best to zero the results then wait a few seconds). Trace the high ports back to the edge switch or device they relate to and run the same test on that device and so on until you reach the very end device. If the utilisation stats show all is normal it might not be a loop, they do tend to just bring everything to a stop. Any dumb switches on the network at all?
