I was having this same problem and ran into this thread. In my case it was usually position keyframes which would move back and forth to get from 1 keyframe to the next. So if I set one keyframe at 0.0 and the next at 30.0, it might dip to -10.0 inbetween them and then go back out to 30 by the 2nd keyframe. Or even stranger, I would have 2 keyframes with identical XYZ positions, but they would slightly move inbetween only to return once I hit the 2nd keyframe.
The solution I found was to right click on the 2nd keyframe (where there is unwanted motion between the 1st and 2nd) and select Keyframe Interpolation. In the Keyframe Interpolation window, you should change the value in the Spatial Interpolation dropdown to LINEAR if it is set on Bezier or Continuous Bezier. I found in all instances of this happening for me, the Spatial Interpolation was either set on Bezier or Continuous Bezier. This does not appear to affect the Easy Ease settings (at least for position keyframes) which are set by the Roving dropdown.