I live in an apartment complex with a single network connection. The address is given to me from the complexes shared router as a DHCP address - this is where the ISP has our complex's modem. My neighbor also has an AirPort Express and I think I may be getting conflict with his... in my AirPort Utility, I can see his "horizontally" in line with mine connecting to "Internet", not vertically.
If I connect straight from my computer to my network wall jack, I can browse to my hearts content... I have a solid speed and all is well. If I plug in my AirPort Express to the jack, I get solid green on the Express as well as on the "Internet" icon in my AirPort utility (hardwire connection from my laptop). As long as my laptop's wireless radio is off, I have a solid connection. If I try to connect WiFi from my iPhone 4 or turn on my WiFi radio on my laptop, I get an intermittent wireless connection and spotty internet.
I also keep getting this DUAL NAT message on my screen.
Let me know if you think you may know.
[10/15/2012 - I will do my best to get a screenshot]