Not sure if you're having the same problem I ran into, but I had a pair of grey lines show up on my menu as well, using the Firefox Web Developer toolbar, I finally traced it down to:

.MainMenu_MenuBreak {
}

I tested this by using what I call a "tracing color"...simply I used the attribute    background-color: #00FF00;    to identify if that changed the grey line to a bright green. In my case it did, and I was able to simply match the color to my separator image, problem solved.

Hope this helps you.

~SB