The internet sharing that I have setup a few days ago was dropping quite frequently. I think I have found the cause and the also the solution to it. It is just simply due to interference. I had never encoutered such problem before. That is the reason why I did not go take a look at the signal and noise ratio.
I have tried channel 6 and channel 9. The signal was about -55 dBm with some amount of noise. But with channel 11, I am getting -49 dBm. -55 dBm is in fact considered to be good signal, the problem is the amount of noise I was getting.