Full Cone NAT using OSX/natd as a router to enable PS3 games?

Hi,
I first would like to describe my configuration:

- MacBook with OSX 10.5.2
- USB HSDPA modem (mobile 3G connection)
- Internet interface ppp0 shared to Ethernet (enabling natd)
- PS3 connecting to Internet via Ethernet, using DHCP
- The PS3 gets NAT3 (it seems it's due to my ISP)

I also tried using a VPN (PPTP) with a Swiss provider:

- Internet interface ppp1 shared to Ethernet
- All traffic makes use of the VPN in network preference
- The PS3 get NAT2

Now the problem: like thousands of people I am unable to play GTA4 online most of the time. All other PS3 internet connections are just fine. One guy in the PS forum said that he solved the problem enabling the "Game mode" in his router. That is better known as the FULL CONE NAT.

Is there any way to mess up with the natd parameters to enable it in OSX as well? I tried using the option "-redirect_port udp <PS3 IP addr>:<port> <port>" but I am not sure about what I did. It didn't change anything anyway. I can't find too much documentation about OSX natd either. The man page isn't enough. Any idea? Some help with natd generally speaking would be appreciated as well. Thanx.

PS: I tried modifying some sysctl parameters as well, without luck. But I don't think at this point they have anything to do with it. monitoring the net it really looks like a NAT problem.