How do I fix overly yeasty kefir? 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?
I can not give you an answer to your question, but whenever I had problems with kefir, I was told to thoroughly rinse the tuber and restart the kefir. 
On this German Website they recommend to restart with 200ml of milk and 1 tablespoon of lactose for a recovery of the kefir. I do not know if this could change the yeast to bacteria ratio.

On a sideline, this Russian website recommend a ratio of 107 CFU/g of lactic acid bacteria to 104 CFU/g of yeast for health reasons. If you don't get the yeast to bacteria ratio down, maybe a fresh start with a kefir with 104:107 could help?