Ubuntu 14.04 wifi unstable

Hi all, I added an SSD to my laptop and installed Ubuntu on it. Everything is fine except that the wifi connection is unstable from time to time. Sometimes, I cannot open any webpage though the wifi icon in the upper right corner is still on. The same issue does not occur in the windows on the pre-installed hard drive.

I tried many suggestions for those who reported similar issues, but none of them solved the problem.

Hope there is a solution or a way to diagnose the issue.