Freebsd 6.1 with ipfilter 4.1.26 ipnat quits working

Hello,

We are having a problem where ipnat quits functioning after some time
period. Using
default values it can happen in a few minutes. With larger values it
might take a day or more.
I have been searching looking for a detailed description of what all
the various parameters do
without much luck.

Can someone point me to a definitive definition of the values and how
to tune them.

Thanks,
Steve

ipf -V
ipf: IP Filter: v4.1.26 (416)
Kernel: IP Filter: v4.1.26
Running: yes
Log Flags: 0 = none set
Default: pass all, Logging: available
Active list: 0
Feature mask: 0x87