icmp problem

hi i have a problem with my icmp, i have a router that
performs nat. i cannot ping to internet hosts from
more than one stations situated behind NAT at once. if
i want to ping from another station i have to stop the
ping that was initiated from the first host, and after
a few seconds i can ping from another station.i've
checked firewll and i have no ipfw rules that could
stop icmp traffic. where should i continue my search
and what can i do to resolv this problem. i really
have to get ping wrking from more than one stations at
once.

10x for your help

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