Per Black Magic forum admin request, posting this here as well:

Just updated to OSX 10.10.5 with new Apple ProVideo codecs, MC 8.4.1 (with Symphony) and BM Desktop Video 10.5.

Same problem as with others on both Mac & PC of very stuttered and tearing footage (my example with 1080p 29.97 .mxf footage).

Tried playing back from a newly created test project and that did not work.

Haven't tried importing a new piece of footage, but shouldn't have to worry about that!

Back to Desktop Video 10.4.2 and playback is fine again on Avid.

FCPX & Adobe Premiere CC 2015 both fine either way.

Its terribly uphill with BlackMagic and their drivers that work, then don't work, work, then don't work.

It's just too unpredictable in a busy Post environment.

We've started moving over to AJA which have always been more stable. You then remember what it's like to have I/O that just works without fuss.

I have the same problem, Blackmagic 10.5 gives very glitchy playback using a Decklink Extreme 3D card. I'm on OSX 10.10.5 and MC 8.4.1.

I'm back on BM 10.4.1 driver now as I had a different problem with 10.4.2 and 10.4.3 - It'd slow MC down when creating a title, importing a file or rendering an effect.

Blackmagic driver is always terribly unreliable with MC, hopefully things improve with the DNXIO. MC is much snappier and more reliable when I use my Nitris DX.

having the same issue on an HPz800 with a Decklink Studio 2, only with MC, After Effects and Premiere works perfectly. 

Same issue  ... any idea or ETA on a fix?

Rolled Back to Black Magic Desktop Video 10.4.1. Is working now. Hopefully 10.5.1 will fix this.

same. rolled back to 10.4.3 but experienced beach balls and hanging whenever media needed to be generated. 10.4.1 works as it should.

Same here. Tearing issues on 10.5, found this thread, rolled back to 10.4.1. Thanks for confirming

I'm having tearing issues on BM Desktop 10.4.1, MC4.2, OS 10.9.5. Should I step back to an older Vers of MC? Don't remember seeing any tearing before updating MC.

Never mind. Installed BM Desktop 10.4.3 and works. Tried 10.5 and it was really bad FYI. 

- update -

After a little more testing I was noticing random color flashing from time to time. Rolled back to MC 8.4.1, BM Desktop 10.4.1 and is working fine so far. No tearing either.