FreeBSD 5.4 Dropping off Network

[Jovan Ross]

I am experiencing problems keeping FreeBSD 5.4 connected. I have uptimes of 
5 or 6 days - sometimes 10 or 11, then, without freezing the machine 
totally, it stops responding to network traffic. I get DOS attacked every 
once and a while and my logs are also filled with failed auths from password 
crackers but it seems that I have the worst effects from it. I don't have 
any web traffic yet b/c this is my development machine. I have portsentry 
with a default open firewall config running. I even took the firewall & 
portsentry off and I get the same problem. I am new to FreeBSD and have done 
the standard security procedures that new ones are advised to do:

enabled secure level 1
syslogd -ss
no portmap
ssh protocol 2
no inetd (could this help my server get up again if it loses connectivity or 
a service fails?)
no ftpd
no ntpd

sysctl:
log in vain tcp/udp
blackhole 2 tcp
blackhole 1 udp
ip rtexpire 2
ip rtminexpire 2
nmbclusters 81920
maxfiles 32768
maxfilesperproc 32768
maxusers 512
somaxconn 1024
tcp sendspace 8192
tcp recvspace 16384
tcp always_keepalive 1
maxsockets 163840
maxsockbuf 2097152

Am I missing something? I want to experience the stability that I've been 
hearing from FreeBSD users but have not been able to achieve it. Could there 
possibly be a setting that says basically: "In case of attack deny all 
connections?" I know I may be stretching it but I've exhausted all my other 
ideas.

Please let me know if you need any information - I will gladly send 
anything.