Either you have a misplaced keyframe causing the additional rotation OR instead of moving the Speed Graph, you moved the Value Graph.
Switch the last keyframe to linear and then check. If all is fine, Then select the last keyframe and right-click and pres SHIFT+F9 to change the linear keyframe to an ease-in keyframe. Check again. If all is well then jump into the graph editor and ensure that you are viewing the speed graph, then adjust the graph accordingly to get an even smoother ease-in.
HTH
RoRK
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