How do I loop my animations seamlessly?

I am trying a complete my method, which crops Maya-Animations (from Maya-Models) to specific ones using the SkinnedModelProcessor.

My only problem is, that after one animation-loop completes, the model stays in its last position for about 2-3 seconds until he catches up to the run-cycle again.

My goal is to get rid of that freeze time. I want to loop it seamlessly.

Here is what I wrote:

public AnimationClip getCurrentAnimation(string clipName, string cropName, float blendTime, int beginFrame, int endFrame, int totalFrames)
{
    //The new list which only has the specific keyframes
    List<Keyframe> region = new List<Keyframe>();

    //The original clip "Take 001" from Maya, which has every
    //animation in one timeline
    AnimationClip clip = _skinningData.AnimationClips[clipName];

    //The new duration of the cropped clip
    int duration = 0;

    //Since XNA makes 7502 frames out of 21, I have to 
    //calc the region-keyframes to proper numbers
    if (endFrame == 0)
        endFrame = clip.Keyframes.Count;
    else
        endFrame = ((endFrame * clip.Keyframes.Count) / totalFrames);

        beginFrame = ((beginFrame * clip.Keyframes.Count) / totalFrames);

    //Start to sort out
        for (int i = beginFrame; i < endFrame; i += 1)
        {
           Keyframe k = clip.Keyframes[i];

           //Add duration of 16 (60fps) to overall-length
           //of animation, if this keyframe does some "real" 
           //transformation, instead of being a static keyframe of XNA
           if (k.Time.Milliseconds > duration)
                duration += 16;

            k.Time = TimeSpan.FromMilliseconds(duration);
            region.Add(k);
         }

         AnimationClip cropped = new AnimationClip(cropName, TimeSpan.FromMilliseconds(duration), region);
         return cropped;
}