1) why do you have the mega build? Any super functions you need? Put mini on if you don't need anything fancy.
2) ditch the mixed mode and choose n or ng, channel with 40MHz. Try above then below.
3) do NOT use chanel 6 try others. Best yet Go to status/wireless/survey and see which channels are free! You can use scanning soft from your laptop like inSIIDEr:
3b boost the power no more than 150mW or 23dBM:
4) Read up on your routers how-to maybe you forgot to disable something. SPI firewall disabled? UPNP disabled? http://www.dd-wrt.com/wiki/index.php/Asus_RT-N16#Current_Known_Issues_and_Bugs
5) if after restart problem disappears no biggie. Go to admin/scripts (can't remember where exactly) or was it cron? Anyways add a restart every 24-hours. Helped me once.
shutdown -r -f -t 0
6) In status check for cpu/ram usage. Se what peaks.
7)your router has broadcom chip what chip does your MAC wrl card have? Sometimes broadcom doesn't play well with atheros and vice versa in dd-wrt. Try another laptop. Try lan connection maybe there's something in the LAN wires.