Laggy _x-movement, need some help!

Hi all!

Im making a slidebased website in flash and there is a problem.
The possition of those black boxes is controlled bij the mouse, but…
Its laggy! the swf is flickering and i dont know why! :puzzle:

Btw, im talking about THIS FILE

Does annyone know how this can be fixed?
The swf has a framerate of 25fps, and it doesnt matter if i in/decrease it.
Also, the motion is done like this:

blackBox._x += extraX;

Ive made the script myself, but im not a pro, so maybe its the wrong approach?
I hope u can help me, thanx! =)