iptables against nmap

Hi list, i've testet SuSE 8.0 with iptables and squid. When i start many (7) nmap scans, options -sF sX sN sS sU, sychronously from another maschine against the public interface of the Squid and iptables firewall, the scanlogd service crashes and the squid is very slow. The users behind the Firewall don't see any Homepage in their browsers. All sites are unreachable. If the scans are stopped, everything works fine. Any ideas, what could happen? regards Andreas Wahlert