tween jumps... why?

I'm having a problem with images jumping once I add a motion tween to them. Not a big jump. Just skip to the left or right and sometimes distorts the final frame. This mainly happens when I try to fade up a bitmat or type object. So in other words... import an image. Convert to symbol graphic. Add key frame 10 frames away. turn brightness on orignal frame all the way down. Add motion tween. Test. {looking for a smooth fade up}.
Fade works fine but the image skips at the end of the motion tween. Why? I didn''t move it. I just added a key frame. Very odd? is this common?

thanks.