If I understand your problem correctly, that's a common result. You're not doing anything wrong really. What is happening is the interpolation between your key frames are taking into account the keys on either side as well to create a smooth spline between your keys.

Open up the graph editor and you'll have to change the tangents of your curves. You'll notice smooth curves in between keys, and they might curve up and down even though the keys surrounding that section have the same value. There are several ways to fix this. One way is to marquee select the keys causing the issue, and select a new tangent type. Tangents > flat, linear, stepped etc.Save a backup of your scene so you can freely experiment.

Stepped is interesting because it will not do any interpolation, it will be at the value of one key all the way until it reaches the next key, then immediately switches. This might work for you if there is a section where something should not be moving.

You can also: select the key of interest, marquee select a tangent handle, select your move tool (w) and MMB drag to change the angle manually.

Also you could try breaking the tangents so each side behaves differently.

I think you get the idea that you get HUGE amount of control over your animation in the graph editor where you can modify your tangents.