Perp Frame Rotation

Hi, 

I'm trying to create a parametric model where a rectangle at a given point on a line (point on curve component highlighted in the definition) is always perpendicular to another line starting at this point and ending at a fixed point. Both the lines are planar. 

I seem to have managed to set this up to a degree, however when the point on curve reaches a certain point both the perp frame and my rectangle rotate instead of remaining planar. (This happens towards the origin, it is easy to see by manipulating the slider). 

If anyone has any ideas why this is happening, or any solutions it would be greatly appreciated. Definition attached

Thanks, 

Tom