[ale] iptables SNAT problem

[JK]

This is driving me nuts.

I have a device that is sending UDP packets from IP
address 128.2.1.125, thru my firewall, and out the
firewall's eth2 to port 7777 at IP 192.168.1.10.  What
I want is to SNAT those packets so that the receiver
sees them as coming from 128.1.110.104. So on the firewall
box I do:

iptables -t nat -I POSTROUTING -o eth2 -s 128.2.1.125 -j SNAT 
--to-source 128.1.110.104

This rule never fires.  (A similar rule with the "-j SNAT..."
replaced with "-j LOG" also never fires.) I can run a tcpdump
on eth2 and see these **(&%^$ packets leaving with source address
128.2.1.125. I know I had this working before, but I have no idea
how, and I can't really afford to pull out any more of my precious,
precious hair.  Google has not answered this question; it's dead
to me now.  Help?

Thx,

-- JK

PS: AAAAAAAAAARGH!!!!!

-- 
I do not particularly want to go where the money is -
  it usually does not smell nice there. -- A. Stepanov