Few things to look for
1)High CPU on the switch/router
2)Slow user connectivity and slow telnet to the switch
3)High traffic in the backplace(in certain switches like CAT5000/6000, you can do a sh system to see backplance traffic)
4)High traffic rates on ports(check for high counters for broadcast/multicast)
If the broadcast is STP related, the following pages should be useful
http://www.cisco.com/warp/public/473/16.html
http://www.cisco.com/warp/public/473/170.html