Video frames were dropped during playback.
iMac Retina 5K, 27-inch, Late 2015)
4 GHz Intel Core i7
16 GB 1867 MHz DDR3
AMD Radeon R9 M395X 4096 MB
1080p HD 23.98p
External ESATA
Write 163
Read 160
iMac with Retina 5K display, macOS Sierra (10.12.5), null
There are a lot of reasons why frames are dropped. Go into Preferences > Playback and turn off the warnings (under Playback, If a frame drops... and If frames drop...).
Things that cause dropped frames:
Native video (not transcoded to ProRes or "Optimized" media); media that needs conforming; etc.
Adding Effects, Titles, or other overlays; anything that uses keyframed animation;
Background processes
Hardware configurations
If you notice serious slowdowns or skips, take some time to render your media (Control-R for selected items, Control-Shift-R to render all) or if you have Background rendering turned on, wait until it has finished.
What's the external eSATA? Unless it's uncompressed there's a problem that probably needs an engineer to look at.
Video frames were dropped during playback.
