InDesign Key lines on PDF

I have been using InDesign since the CS version, I am currently running CS3. I am getting very faint key lines appearing on my High Resolution PDF files. At first I thought it may just be a rendering problem with low memory or underpowered graphics card, but when I view the PDF in acrobat and run the eye drop tool over the line the percentage value changes. This does show at 100% but the lines are so thin that they fill in when sent to plate so there is no immediate danger, but is there a solution for this problem?