Don’t despair!  You shouldn’t expect a stable starter for at *least* ten-fourteen days. In that time, a lot is changing within your starter. As acids (produced by the Lactobacillus bacteria) build up, they start to kill other microbes. Sometimes these microbes are dominant in the starter for the first few days and can make a starter appear to be colonized by bubbly yeasty goodness, when in fact they are simply other microbes that are producing carbon dioxide.  As acids build up, they will suppress other microbes that are producing carbon dioxide – that is until the acid tolerant yeasts have a chance to build up as well. So dont give up yet. Keep discarding and feeding your starter!