I had a similar issue. I tried upgrading to 15962 and it didn't seem to help. What did seem to help was manually setting the channel to 161 - 5.805 GHz. Other channels might have worked but I saw this one listed as a good choice on the main dd-wrt wiki page for the E2000.
Here are the settings I'm using that are working:
Net Mode: N-Only (5 ghz)
Channel: 161 - 5.805 Ghz
Width: 40 Mhz
Control: upper
SSID Broadcast enabled
Range: 2000
Network Bridged