Slo-mo Malfunction

Tho I've had my iPhone 5S for a year and a half I only just used the slo-mo feature. It was odd. When played back it seemed normal speed for a little while and then began to run in slow motion. After a while it sped back up to normal. This was borne out visually by the timeline as it was depicted with an arbitrary scale with points closer and farther apart when the speed change.

Moreover, and more troubling, I trimmed the two ends of the clip and then posted it to Facebook When played on their viewer not only was the video NOT in slo-mo, it in fact seemed slightly sped up.

What's going on here? I'm running the latest iOS.