[F8] Dynamic line opacity

[rockonpeaceout]

I have made this "application" in flash.
Make the pen size quite thick and set the line opacity to low and draw with the pen tool.

As you can see, it does not draw completely opaque, there are dots of solid (or more solid) colour in the line. Here is the method I used to draw:

mc.moveTo(_xmouse, _ymouse);
mc.lineStyle(_root.lineThickness, _root.firstColour, _root.lineOpac);
mc.lineTo(_xmouse, _ymouse); 

I have also tried drawing at 100% opacity and then changing the opacity of the "mc" movieclip and the same thing happens. Is there any way to stop it doing this and make the whole line the same transparency?

Thanks