You guys are starting to give me a headache with those multiple posts over four separate threads about NAT types...
Try the following script (can be set as a nat-start script, or manually run):
Code:
#!/bin/sh
iptables -t nat -D POSTROUTING ! -s $(nvram get wan0_ipaddr) -o $(nvram get wan0_ifname) -j MASQUERADE
iptables -t nat -I POSTROUTING -o $(nvram get wan0_ifname) -j SNAT --to-source $(nvram get wan0_ipaddr)
The PREROUTING DNAT rules are already configured by UPNP. What this does is change the POSTROUTING MASQUERADE into an SNAT.
No guarantee it will solve your problems (because there's no guarantee this IS the source of your problem).
BTW, Moderate NAT is not the end of the world. What is problematic is if you get a Strict NAT.