Hi,
It looks like you have accidently added the same motion tween twice at different levels of nesting within the same symbol. Also within nesting, you're using Movieclip instance somewhere and Graphic instance at other places.
This answers both of your queries:
1. Speed is different because the resultant speed is the addition of two motions along the same path. (When previewing within application, animations within a movieclip are not shown but within Graphics are shown.)
2. The disappearance happens due to the same eason. The start and end point of a grapic animation is dependent on the parent timeline and remains the same during looping. Movieclip animations on the other hand, always play completely and may spill over multiple loops. So basically the start point of the animation is getting shifted with every loop.
You can remove one of the tweens from the nested symbol to fix this.
-Nipun
