performance of libnet-based tools

I've been experimenting with a few different
libnet-based tools such as Packit and ISIC.  If I try
to generate a high-volume of traffic using these
tools, with various options and protocols, the error
rate is extremely high.  ISIC says:

Failed to send packet: No buffer space available

The synflood tool that comes as a libnet sample works
for several seconds and begins reporting:

libnet_write: libnet_write_raw_ipv4(): -1 bytes
written (Host is down)

I'm not sure where the problem lies and I would love
to hear any suggestions.  I'm using a 3.0Ghz P4 with
1.5G RAM and a $20 Linksys FA311 card on FreeBSD 5.3
and a 1.3GHZ AMD laptop with 512MB RAM and a built-in
Realtek RTL8139C+ card on FreeBSD 4.9.  

Do I need to tune the OS?  Do I need a better network
card?

I'd like to be able to perform various stress testing
exercises on my test network.  

Thanks.

-Steven