Help with fade in and out on ._visible=true;

In the following code I am controlling the visibility of a MC with the following code:

on (rollOver) {

if (_root.navorange._currentframe > 1){
_root.testthing._visible=false;
}

}

on (rollOut) {

if (_root.navorange._currentframe > 1){
_root.testthing._visible=true;
}

}

This is working fine but the on/off transition is a bit harsh. Is there a way to add a fade in fade out effect with action script so the change from visible to not visible is not so abrupt?

Any help would be greatly appreciated