I have very similar issues. I just upgraded to the Arris dg1670a which is in bridge mode. I have the newest Apple Express router connected to it which is under my desk where both my computers are located. The network is up and works but the speed difference now is quite large. Before I got this modem I was getting 90-100+ Mbps over Wifi to my Macbook Pro and around the same on my Mac Pro. Now I'm getting 25-30mbps over Wifi to the Macbook Pro and 325+mbps wired to the Mac Pro(connected directly via ethernet to modem).
I've contacted Time Warner three times already and they just say it's something to do with the Apple Router. I would believe them if I did not have great speeds before.
The tech I spoke with today actually said they just received an email regarding issues with Apple Express routers but he said they were only getting a max of 130mbps, still far better than what I'm getting.
Please let me know if anyone has any advice. Are there any modem settings that would hinder bandwidth that much?