Line Quality issue

Having an issue where I have graphics that are scaled by a database value. Each of the graphics have an instace of a background object that changes color overlaying the graphic. The blending mode

for the background object is overlay with cache as bitmap checked. 

It seems that occasionally some of the lines disappear in the overlayed line graphic in the flash player.  However if I change the quality to medium

in the flash player it seems to fix the lines. I don't want to do this perminantly because

changing the quality to medium has other ill affects. Does anyone know what is causing this or how to fix it?