HELP! how to smoothen motion tween

it’s not the frame rate problem, and it’s not the slow computer or anything like that.
take a look at
(its 990 bytes)

the animation is just not smooth.
I know there is a solution, because i found it once on the net, but lost all data on my comp so i don’t have the source file nor the bookmark of the smooth tween.

can somebody please help me?