internet connection failed

Our company share 4m bandwidth internet connection, there are pix firewall and router between our internal network and internet connection. Almost 60 users connected to internet simultaneously.

Sometime, user can not connect to internet, symptoms including can not display web page, can not send or receive mail, but succeeding in ping website, not all client encounter this problem at the same time.
I check pix firewall cpu usage and connection number, it is normal as well as router.
I think the reason maybe network overloaded. I want to know who eat up bandwidth.
Does anybody know how to solve this problem?