I had a problem similar to this when I used Xv from XawTV. But I only had this problem when I was using th stock "nv" driver from XFree86 4.1.0. As soon as I started using the "nvidia" driver, the problem went away.
Is running mplayer without Xv an option? This might slow things down, but if you use "-vm" on the SDL drvier and have the appropriate video modes, I don't see why it wouldn't work. (Assuming it's possible to disable Xv)