You need to pay attention to the handles at the start and the the end.
I dare say that your x curve is slowing down and speeding up at the start/end because that is what the automatic handles are supposed to do. They smooth the animation from the keyframe before/after it. However, if there is no keyframe before or after and that curve cuts off halfway through the sine wave shape, it will flatten out.
If that is what you are talking about, one way to fix this is to add a cycle modifier so it knows to keep it smooth.
Or by manually rotating the handles to control the tangent of the curve at the start and end.
Hope that makes sense, Curtis.
