The S4 has a "feature" where it checks your internet connection and decides if it is "good enough" or not. When you get that particular message, the S4 thinks that the internet connection isn't what it should be. This works in conjunction with a setting that will automatically switch your device to carrier data if it deems that your connection is too slow. I get this "unstable" notice quite often at the office where the wifi is sketchy. The wifi still works fine. I would recommend not setting the auto switch function on unless you have unlimited data, because it will frequently switch you from wifi to carrier data. It doesn't tell you it is switching, so you use carrier data unless you notice that it has switched.
