I'm not sure what you mean by "jumps of speed", but...
If you're talking about the fast portions of the video - did you preview the video path before rendering? I find that the path system tried to guess how much time you want between waypoints, but sometimes doesn't guess correctly and you wind up with particularly fast portions when the camera position doesn't move much between the waypoints. Previewing the path can give you a good idea of which areas are too fast (though depending on your machine the preview will overall still be slower than the final rendering), and you can adjust before rendering.
If you're talking about the subtle but still perceptible "skips" that happen periodically - I had this problem with my machine, and I think it's simply an issue with the power of the computer. I bumped the quality down a notch and that problem went away. You can also adjust framerates to "lower quality" without actually lowering the quality - in most cases, you shouldn't need more than 30fps anyway.
when i preview it , it looks ok :\