Allowing Drag ONLY along X axis

I’ve been trying to allow a MC to be draggable ONLY in the X axis.

currently the drag is triggered onPress and stopped onRelease
with an if statement within an EnterFrame triggering the “lock” to Y axis, as I want to lock the MC to X axis until (say a value of 400) then it is freely draggable.
but it gives a “jumpy” effect (i.e. you can see the MC shift and then shift back to it’s “locked” position). From what I figure it’s because there is a slight delay in the “testing” of the if statement.

what is the best way to achieve a draggable MC that is ONLY draggable along the X axis?

Cheers in adv.