I've got the same problem: When I've got TwinView enabled (with a Tv as second monitor) the screen (both on the CRT and TV) gets horizontal lines when playing movies (divx) with lots of motion. To me this looks like some kind of sync-problem, but I'm far from an expert.
Also, when I only use the TV (without TwinView) everything works just fine!!!
Anyone got any ideas?
(Using GeForce 4 MX 400)