The movie has not fully loaded when the code is trying to tell it's timeline to gotoAndStop(5). You can account for the load time with a delay before calling the code. There are two options that come to mind. The first is using an onload method. Movieclips have an onload property to determine when the external movie has loaded.....
The second option would be to check the holder movieclips width or height, assuming that your using an empty movieclip to load to. Once the external movie is loaded, the empty clips width and height are no longer 0. You can use this to control the clip.
Much of it will depend on the structure of your project, but the overall problem your describing is that your trying to control the loaded movies timeline before it is completely loaded.
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