Well, this might not fix it entirely, but it should lessen the effect.

In the myOver function, replace the starting parameter of the Tween (0) with

bg.cover_mc.alpha

And in the myOff function, replace the starting parameter of the Tween (1) with the same thing

bg.cover_mc.alpha

All that means is, when you roll onto a button from another, it'll start a Tween to full alpha, but it'll be referring to cover_mc's CURRENT alpha as its starting point, which should still be 1 (from the last button)