Hm.   I haven't had that problem.   Here's my ipnat.conf:

#
# Redirect Real Networks streams to 10.0.0.9
# (interestingly, it seems to work for any host)
#
#rdr ppp0 0/0 port 7070 -> 10.0.0.9 port 7070 tcp/udp
#
# Redirect ssh to polka (mapped from port 6022)
#
#rdr ppp0 0/0 port 6022 -> 10.0.0.6 port 22 tcp
#
# Redirect ssh to bald (mapped from port 6023)
#
#rdr ppp0 0/0 port 6023 -> 10.0.0.8 port 22 tcp
#
# map all connections from 10.0.0.0/24 to our dynamic IP
#
map fxp1 10.0.0.0/8 -> 0/32 proxy port ftp ftp/tcp
map fxp1 10.0.0.0/8 -> 0/32 portmap tcp/udp 10000:20000
map fxp1 10.0.0.0/8 -> 0/32

I don't claim to know how this works, but it seems to work fine even
when the globally-routable IP address changes.

			       _MelloN_