Firstly, I'm on Mountain Lion 10.8.5 on a Mac Pro 4,1 with 32 GB RAM, a Blackmagic Decklink Studio 2 with Desktop Video 9.9.3, After Effects 12.2 and Premiere 7.2.1.
This issue I'm having is that I am experiencing inconsitent performance with Quicktime Video Preview in After Effects via SDI out to my external video monitor. Video preview will function as expected and then if I switch from AE to another application and then switch back, my external monitor will go black and when I drill into the "Video Preview" settings in AE, the "Output Device" drop down will only list "Computer Monitor Only" as an available device. Sometimes quitting and restarting AE fixes it, sometimes I have to power cycle the machine to get the menu options to come back.
At first, I believed the problem to be linked having video preview turned on in Premiere along side AE and I have since completely disabled this function in Premiere for the current project I'm working on, which delivers as 2048x854 (which for some reason Premiere is incapable of displaying on my external monitor unless I nest the 2048x854 sequence into a 1920x1080 sequence, which is ridiculous considering both After Effects and Resolve are both capable of downscaling the image for playback on 1920x1080 video cards and monitors, but I digress).
Disabling "Blackmagic Playback" in Premiere hasn't fixed the issue of inconsistent playback in AE. As a side note, when playback via SDI does work in both AE and Premiere, the two apps seem to fight over control of the external monitor, one not giving up control when switching back and forth between the two apps. I have checked and unchecked "Disable video output when in the background" in Premiere and neither seems to have any effect. One would hope that these two pieces of software would have a bit more compatibility with the hardware they're attached to when switching back and forth between the composite I'm working on and the edit where the composites live. I never had this kind of issue switching back an forth between previous versions of AE and Final Cut Pro.
Any suggestions or guidence would be greatly appreciated.