The problem is with AgpArt being compiled into the kernel. I realized that when I didn't have a problem with lines was when I had agpart compiled as a module. So you can recompile and make agpart a module or set "NvAgp" to "1" which uses the nvidia driver agp rather than the kernel. That worked for me, so I have agp and no lines. Let me know if this helps - I'm very happy to have finally fixed this annoying problem that has been bothering me for months!