What you're actually seeing when you view the preview monitor is a fairly low quality example of the final rendered video. It could be that your graphics card, processor or memory is struggling to keep up with what's going on. It's not unusual for preview monitors to flicker or have unsteady playback. I used to have an Intel Celeron 1.8 which did this all the time when using Premiere Pro. With some editing software you can render the video and effects as you go, but I don't think Movie Maker has this option.

If it's a memory issue, you could possibly assign more memory to the graphics card to try and eliminate this, if there's an option in the computer bios which allows this.

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