How to detect high bandwidth causing slow Internet connection

All of sudden, Internet connection is really slow. File transferring or application access within LAN is no issue. Having 20 users, there is OK for no Internet connection temporarily. The network structure is simple, Internet > Cisco 837 ADSL router > Cisco PIX 501 firewall > 3COM 24 ports normal switch > 20 hosts.

With only one host direct connect to PIX 501 firewall, the Internet connection is fast, PING result shown avg 20ms.

With only one host direct connect to 3COM switch alone and unplug all other RJ45 cable, the result is same as above.

With all hosts connected to the 3COM switch, the Internet connection is really slow, and ping result shown above 1000ms.

After works, all employees gone home, the Internet connection is fast, ping result shown avg 20ms.

I am not quite sure which host is utilising high bandwidth causing the Internet connections is damn slow.

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