To render if you have added video effects means that the video will have interrumptions, and will not be uniform. To solve this:
Create an extra video track. Drag a static image onto it, and drag the ends of the static image so that it covers the entire length of the project. Then fade the image out so that it can’t be seen (with the opacity control of the track). The presence of this image is enough to make Cinelerra re-compress the whole project and you will get uniform looking output. You may want to save this for the final render of your project, however, as the re-compression can be very slow.