When you play the timeline, the viewer looks like frames are dropped. It's choppy. However, playback indicator is solid and output to an external monitor looks fine. This one happens when the mixer is open. When it's closed, both the viewer and output appear fine when the timeline is played.
If you set the layout to Full Screen Timeline, so that the viewer and mixer are on the same (secondary) display, then playback looks smooth in the viewer.
Mac Pro 2009 12-core 32GB RAM. Sapphire 7950 GPU. Dual displays plus Decklink card connected to external monitor. Mac OS X 10.11.5. Although I notice this on other machines as well.
Also a lot of sync discrepancies depending on whether you are using the system audio, the Blackmagic I/O device, and even this is intermittent. Hard to edit to a sound cadence. Yeah, I know, you're supposed to line up the cuts with the waveform.
"I always pass on free advice -- its never of any use to me" Oscar Wilde.