Strange behavior(internal comp. pinging)

We are using 512 kbps ADSL line to connect our department (about 50 computers) to the internet through a win2000 server supported with ISA server. The problem is that, sometimes, some internal computers (Win 2000 server or prof. and WinXP ) send large amounts of ICMP echo requests to IP addresses outside our subnet (130.10.0.0 , 255.255.0.0 ) causing the download rate to degrade to unacceptable levels (sometimes to 10 kbps) or even crashing the external network interface . For example, one computer may ping IP addresses from 130.7.0.1 to 130.20.255.254 successively without waiting for replies for the sent requests.

Thanks for your help