Unlike it's big brother the Inspire, the DJI Spark creates a variable frame rate video file. The video stream is encoded using two different rates or rather frame durations: 33.359 ('fast') and 33.438 ms ('slow') and adjusts the very last duration to suit an overall frame rate of exactly 29.97 fps. After the first twenty-seven or so frames the video frames are written in groups of either nine or ten 'fast' frames plus one 'slow' frame. This is evidently done to match the 29.97 fps rate. These spark generated groups are, in total, just a few microseconds out from the duration time of ten or eleven constant duration frames at 29.97 fps.

If a Spark video is loaded into iMovie or Final Cut Pro without making any setting adjustments the editor will generate a constant video stream at the overall fps recorded in the file - 29.97 fps. Because of the missmatch in constant display versus variable recording, each of the the first ten frames in a group of eleven will be displayed too slowly and the 11th too quickly. This gives the characteristic stutter rate of about three stutters a second. Could the stutter be caused by the bit rate being insufficient to correctly display the 'slow' frame?

iMovie's handling of the video appears to be considerably improved, but not completely, by setting the frame override to 99% of 'normal', although on a large screen a much slower but still detectable stutter can be observed.

Using Handbrake to 'pre-process' the video by converting it to a constant frame rate of 29.97 fps and a bit rate of 24,000 kbps seems to greatly improve the quality in iMovie.

In Final Cut Pro you will observe the same behaviour. However in that software it is very easy to use a custom setting and to determine the suitable frame and bit rates which appears to work.

This work was made possible by using the excellent free apps FFmpeg and MediaInfo on my iMac running High Sierra and examining the video packet data.

DJI are aware of this issue.

This is the exact type of BS none of the reviewers or even DJI own up to. Having to jump through hoops to edit a simple video is not acceptable given the price point. I hope more people expose this flaw and DJI come to terms in fixing this rubbish. Yes I own a Spark and at the moment is useless if you cant render decent video from it.

That seems like a complex and inefficient way to write data. Is there a particular reason for it?

I haven't noticed any stutter in the Spark videos I see on the internet. Has greater compression smoothed them out somehow, or have they all been corrected first? I was about to buy a Spark as a flying camera, but this is now giving me second thoughts.

I was lost at the first paragraph but from what I got out of the post, there is some inconvenience for getting the best possible footage with the DJI Spark?

Is there nothing to the OP, then?

Great explanation.  Thanks for that.

I found the best way to fix this in iMovie is to import a Sample 30fps video first.  Add that to your timeline, then add the other clips and remove the the original sample file

Yes it’s an “extra” step, but. It takes probably 10 seconds, if that to do this.   It’s now just part of my workflow.

I posted something on this forum a couple of weeks ago on how I fixed it.  
https://forum.dji.com/thread-130459-1-1.html

djiuser_GJaqn2xelynI Posted at 2018-2-9 17:23
This is the exact type of BS none of the reviewers or even DJI own up to. Having to jump through hoops to edit a simple video is not acceptable given the price point. I hope more people expose this flaw and DJI come to terms in fixing this rubbish. Yes I own a Spark and at the moment is useless if you cant render decent video from it.

You can render decent video...the evidence is all over this forum

This issue only impacts iMovie really, and there is a simple fix for that.

I agree it should be fixed, but it adds an extra 10 seconds of time to my workflow, so it’s not that big a deal.

I've been using Edius video editing software for "Grassvalley" (formerly Canopus) for years. This is perfect because it doesn't matter to the frame number, it automatically (interpolates) the selected format. The slogan of the company is "Edit anything fast".
It has worked for me and there is no problem even if there is 25fps HD, 29.97 HD, and 50fps UHD (Osmo pocket) in the same project.