This model sounds like the one my former neighbor had. He complained about it to me last week and I walked him through the firmware upgrade.
What happened? The problems are gone and he's not shopping extenders now.
Try that!
Does somebody know specifically what type of Antenna Connectors does the ASUS RT-N66U uses ? Does it uses RSMA, RP SMA or other type of connector ?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic