I am using Blender 2.74 with LUbuntu 14.10. In this case preview and rendering acted different, as I explain below. I tried also in Blender 2.69 and in this version preview was showing the same behaviour as rendering, which is what I am trying to resolve.
I've got a project with 6 channels. Channel 1 is just for Color strips, to easy the process of fading in and out. Channel 2 is a Movie strip. Channel 3 is the (processed) audio of that movie and channels 4 to 6 contain different Scenes with text fading in and out.
The movie and its related sound also fade in and out. I play with Keyframes and opacity for all fades. When previewing everything runs smooth.
But when rendering, with no apparent reason the movie strip fades out to black just a short time after its programmed fade in, and stays that way short before ending, where it fades in again (by itself) before performing its programmed fading out.
If I move that strip from channel 2 to channel 7, then its channel 4 (i. e., one of the text scenes) the one fading in and out with no apparent reason.
I've checked Keyframes in Graph Editor for the different strips and I can't see anything abnormal. All the strips have an Alpha Over blend except the color strips in channel one, with a Cross blend.