wrong bandwitdh

Hi,
we try nfa 5.5 build 5505 with a cisco 7600 12.2(18)SXF7 with this lines:

ip flow-cache timeout active 1
ip flow-cache timeout inactive 15
ip route-cache flow

and in all interfaces:

ip flow-export version 5 xxx.xxx.xxx.xxx

where xxx.xxx.xxx.xxx is the ip of nfs server.
All works right but the measured bandwith per second is really wrong (near 1:100).
We try also with a cisco 12000 but with the same result.
I don't know if are too much data to process (cisco 7600 are near 700-800 mbit per interface, 5-6 interface), but nfa server are not busy (0.1 load, linux or not so busy windows machine the result is the same).
What can we do?