If the 'AIR' led goes out during the night (is that the case?)
you should open the webconfiguration page of the wl500gp.
goto 'wireless' -> 'advanced' and check 'time of day to enable radio'
I am assuming that you use the 'oleg' firmware.
Which firmware version are you using?
Is the 'AIR" led on or off when you see the problem?
-Try using a fixed channel instead off auto e.g. channel 6
-Resetting the router might help