Thanks FurryNutz, I will follow your instructions, but first let me provide some information:
my router HW ver: A4, F/W ver: 1.32NA (factory), ver:1.35NA updated
My internet speed is 10mb, but with the wifi I am getting only around 7mb which is OK, however, sometimes when I watch Youtube I get buffering from time to time which makes me feel that my internet speed it going down, and another test with a WiFi analyzer app for android, it has a meter that is not stable, so it is going up and down form -55 to -65dbm, although I am not sure how accurate is this app, another info is that the best channels are 12, 13, and 14, but they are not available in the router, and I live in a buliding which has a lot of routers with channels that are overlaps with mine.
I just found a new F/W 1.37, I will try it.