I also have troubles with slow motions. My vegas doesn't make lines, but it pixelates. Every time i try to make a slo-mo, whether is by expanding (ctrl+clicking) the event, or by adjusting the velocity envelope, the result's the same when I render or prerender. I can even see the pixels while previewing the rendering action.
Another strange thing is that when i preview it normally, I mean, with out rendering, it looks just perfect.
Somebody help me please!