Sotgun has sound advice. I'm on DSL with Bell and used to have it on a business model router with an integrated modem. After a reset it would be NAT2 open and after some uptime it would swap to NAT 3 strict for some unknown reason. Security setting or somesuch must kick in i'm guessing... I got sick of it and went and bought a new router and used a real DSL modem to establish the connection.
Always NAT 2 open now. My PS3 is NOT in the DMZ and UPnP is enabled.
This is what i bought:
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Cisco bought out Linksys a while back and made it into their "home line" products. The Cisco is more for businesses and is more oriented on security. I wouldn't worry about decreased security features since i doubt china will try to hack you to get some national security documents.
My suggestions:
1- Buy a Linksys router. Keep the bill since you should have 30 days to return it if it didn't change a thing.
2- Connect the PS3 via cable if you can. It's more stable and will offer higher speeds than a wireless g connection can. 54 mbps vs 100 mbps.
Optional (I didn't need to do this):
3- Assign a static IP to the console.
4- Enable and add that IP to the DMZ.
5- Disable UPnP if you really need to. Some games will require this so it might cause more issues than be helpful in the future.
If your really concerned about security, you can disable remote administration so you need to be hard wired to the router to edit setting and if your a freak, you can control access via MAC address...
Is your PS3 connected wireless? If so do you have any other wireless devices connecting? Do any of them use the "B" or "N" technology? If not, you could get rid of mixed mode compatibility in the router if you don't need it, it will help with stability forcing it to one type.
You could also check and see if any other devices are running on the same frequency (ie.: cordless phone on a 2.4 GHz) and change it to a different channel within the router.
Hope this helps. Good luck.