erics already asked the obvious - some routers will start to drop the connection once it gets below say 6dB.. the Netgear should be fine though and normally hangs on way below that.
The fact that it does drop IMHO would seem to indicate a very sudden noise burst from somewhere. - Someone switching something on for eg.
Although more users can cause more cross-talk (maybe a dB ) it certainly shouldnt affect it to the extent you are experiencing.
The steady drop from 11db to around 7db sounds like "normal evening drop".. it should however recover by the next day, it may just be your router not "keeping up". As long as the next evening it doesnt drop a steady further 4dB then it sounds like it could just be the behaviour of your particular router that it isnt showing when it goes back up:/
