Pixel C Slows at approx. 29% Battery

When my Pixel C battery level falls to 30 or 29%, it begins to run very slow. Operations that normally happen in fractions of a second, slow to several seconds (pages loading, changes in game screens, etc...). Then approx. 10 minutes later, the power falls to zero and it shuts down, with no warning. I've upgraded to Android V7.0, and there is no change. Any advice?