When you import a BVH into Poser, every frame throughout the durration of the animation is keyframed, leaving Poser no room to interpolate any of the animation. When you are slow down your animation, you are spreading your current keyframes over a larger area of frames. This will give you frames in your animation that aren't keyframed, and the little gnomes inside your computer get hard to work on iterpolating or 'tweening' the areas between your keyframes.

The problem lies in the fact that because Poser uses spline interpolation by default, and there are so many keyframes, it causes little loopty-loops in your animation path. Spline animation does great things with a few keyframes over a long period of time, but because the keyframes are still so close together you get these quirks.

To fix it, open up your Animation Palette, drag your selecter to select all of your keyframes in the animation and then hit the orangish "Linear" button in the controls on the upper right side of the palette. Now movements will be in a straight line from keyframe to keyframe, but with so many keyframes you'll probably still have very smooth motion in your animation.

Hope this helps.