I had a similar problem not too long ago. In my case it turned out that the freehub had gotten gunked up and at first would only slip when I was on the 11t cog. As I continued to ride on the next cog and on others up hill it slowly started to slip on those as well but that took time before it started happening. What I did was pull the freehub off and take it apart as best I could and flushed it with mineral spirits. After it dried I relubed it and it has worked fine since.