Problems with setMask and cacheAsBitmap

Can anyone shed some light on opacity mask max width? I have discovered a max width of a movieClip that is masked using opacity to around 2800+. Masks w/o opacity don’t seem to have this problem.

holderMov = movieClip to hold stretchyMov & masker
masker = movieClip with gradient black to black/opacity 0%
stretchyMov = movieClip that is being scaled.[INDENT]**[SIZE=2]holderMov.stretchyMov.cacheAsBitmap = true;
holderMov.masker.cacheAsBitmap= true;
holderMov.stretchyMov.mask= holderMov.masker;

holderMov.addEventListener(MouseEvent.MOUSE_UP,onPresser);

function onPresser(evt:Event){
holderMov.stretchyMov.width = holderMov.stretchyMov.width + 100;
trace(holderMov.width); [/SIZE][SIZE=2]// at around 2800+ mask no longer works = no mask
[/SIZE]
[SIZE=2] }

stop();[/SIZE]**
[/INDENT]