This problem is two fold and I've experienced it for years. Now I'm using a much faster system that is handicapped by not being able to use multiple cores. I had hoped that by now this would just work.
First, when using multiframe rendering AE often hangs attempting to initialize background processes. As often as one RAM previews this can be quite a problem since it grinds all progress to a halt. The solution is often restarting AE. Checking Activity Monitor during this time shows that Adobe QT32 server is non-responsive and a whole line of the AE background rendering processes doing nothing. I have not found a common element in projects that experience this issue, it can happen just with text on a solid.
Secondly, when using multiframe rendering an entire sequence can ram preview, but then disappear as soon as it finishes. Often after this issue the first issue will pop-up.
A little about my setup:
Late 2013 Mac Pro 8-Core
64 GB Ram
2x AMD FirePro D700
OS X 10.9.4
AE CC 2014 Updated to the latest patch
52 GB RAM available for AE
Render Multiple Frames Simultaneously - ON
Only for Render Que - OFF
RAM per CPU - 4 GB (problem persists when set to 6 GB)
Until this feature actually works reliably I'll have to go back single frame rendering, which is a bit like owning a Ferrari and having a transmission stuck in first gear.
I don't think that's the issue since it doesn't generate the 'needs 2 or more frames' error. It's not even using it's full allotment of RAM when this issue happens.
Are you still having this issue? What you describe is EXACTLY what is happening on 2 Dual XEON systems we recently sold. Except, these are systems running Windows 7 with an NVIDIA Quadro K6000 graphic card.