What OS are you using?
Dock a panel (like the unused Progress panel) to the bottom of your composition panel. Does that help?
I'm using OSX 10.12.2
Tried docking the Progress panel but no difference
Apple fixed some AE-related bugs in Sierra in a recent update, you might consider updating to 10.12.3
If that doesn't solve it for you, we're going to need a lot more information from you. Stuff like your composition settings, what's in them, how you're initiating the previews, etc.
I updated OSX to 10.12.3 and still no joy
I initiate preview by clicking the Preview button on the Preview Panel
What do you need to know about my settings? etc
What frame rate? What frame size? Do you have your Composition Panel resolution set to auto and the Magnification factor set to 50% or less?
I'm running OSX 10.12.3 on a half dozen machines and I'm having no problems at all with HD footage at half (auto) resolution, 29.97 fps and 1080 or even 4K resolution. Well, ok, the mac mini won't playback 4K at half resolution and 29.97 fps, but iMacs and MacBook Pro's are doing fine. The length of the preview depends on the available memory.
If you are having constant playback problems with 1080 projects up to 29.97 fps then you are starving the system for resources some how. Don't expect to play back HD at 60 fps on anything but a machine with a bunch of power and a very fast drive array for your cache.
My frame rate is 25fps
Comp Panel resolution is set to a quarter
and mag factor usually 50%
I'm on Mac Pro
3.5 GHz 6-Core Intel Xeon E5
24 GB 1866 MHz DDR3
My cache drive is SSD on USB3
What's the comp frame size?
You should have no problems unless you are adding some footage that is giving the CPU problems.
1080 x 1920
Never had this problem before and no change to my setup apart from AE updates
Were you ever able to solve this issue? If so, how? Please let us know the status of your project. We're here to help.