The AOSS feature randomizes the encryption and SSID of the wireless network, using a mixed TKIP/AES encryption method. That may be what is causing your speed loss. Turn off AOSS, and use AES encryption to resolve this.
To address your PC not being able to connect to the internet, log into the web interface of your router and verify that the Internet/LAN option->Method of Acquiring IP address is set to "Acquire an IP address from a DHCP server" not "Perform Easy Setup." This is, of course, only if you are using the User Friendly firmware.
If you are using the Buffalo branded DD-WRT firmware, you'll log into the web interface and verify that the WAN Connection type is set to "Automatic - via DHCP."