My milk kefir grains now produce kefir with a very unpleasant yeasty/bready flavor.  I've tried feeding it for a while, on the assumption that the yeast/bacteria will get back into balance, but this has failed.   I'm worried there may be cross-contamination from sourdough starter.   Is there a way to fix this, or do I need to obtain fresh kefir grains? 
Your options are either to change the milk or cover the taste with any type of sweeteners like sugar honey cinnamon nutmeg. If you make smoothies you can add all types of fruit. Don't worry about the sugar intake as the kefir should be suppressing your candida. 