Airport Express to Cable Modem issues

Here is my setup and problem. I have 3 apple computers wirelessly connected to an airport express which is connected to a cable modem (RCN). I plug in the modem power wait for all lights to show green and finish boot up, I plug in the airport via Ethernet and the airport shows a green light (working). All computers internet connections work for about 3 minutes then the internet drops.

I have run the network diagnostics check (Assist me) and the airport utility all yielding the same results.

I have tried just using one computer to test the connection same results. Given these issues I would expect there to be an interference issue but with Comcast I didn't these problems.

Any thoughts I really need to get my internet up today!