What are your Memory & Multiprocessing settings?
If disabling Render Multiple Frames Simultaneously multiprocessing makes everything work OK, then I suspect that the problem may be because of settings that are over-allocating resources. With 8GB of RAM, you are just on the cusp of having enough RAM to use Render Multiple Frames Simultaneously multiprocessing effectively, and you have to be careful with your settings.
There are recommendations for such settings linked to from here, along with a lot of other information about memory and performance.
I started with the defaults (I think) which was 2gb reserved for other applications, 0 cores reserved for other applications, 1gb assigned per core.
I then adjusted somewhat per the advice, moving to 1 core reserved for other applications, 1.5gb assigned per core. Then changed to 2 cores reserved for other applications still with 1.5gb assigned and 3gb reserved for other applications.
The latter reduced the time spent on 'Background Process Status: Loading Projects...' a lot. But the instability was still consistently present (not that I've done a lot of testing though. Just three or four attempts). I haven't tried any further combinations yet.
Again this is only in this large project and only serious when RAMpreviewing with audio (which is a single five minute 44.1 wav file in this case).
I'm sure it all contributes (amount of RAM, project size etc) but it's interesting that the audio is what seems to push it over the edge.
Anyway, I'll use multi frame rendering sparingly with this one.
Audio can cause a special problem for RAM preview. Try muting the audio in the Preview panel and see if you still have the problem.
Impossible to tell the exact cause, but I would assume your audio file is causing "mixing" rather than AE being able to play it immediately. This may be further compounded by some audio effect using a temporal offset to delay the audio and/or even some otehr temporal stuff like time-remapping getting involved...
Sorry about the delay.
Todd: Yes, disabling the audio (using those two core settings I mentioned) makes it actually start rendering a lot quicker (and the rendering is noticeably faster with multiframe switched on). There is a still a period where the program seems to halt while multiframe rendering, I assume, boots itself up. But I suppose that's normal. RAMpreview rendering with multiframe switched off begins almost instantly, but it wouldn't have to do any special reorganisation of data in that instance I guess.
When it did manage to render in multiframe with the sound on, the sound and the picture were often jittery and out of synch. It does seem to be the audio it's having trouble with, yes.
If it's a known problem then I guess it can't be helped much.
Mylenium: Hmm. I'm not sure I read you right, but for the sake of it I'll say there aren't any effects on this audio track. The project audio settings are the same as the file itself so it shouldn't be being resampled. A decent amount of my footage does still have embedded audio but it should be switched off and all the layers in the master timeline have their audio switched off, so hopefully it's being ignored even if any is on by mistake. That may not be how it works however.
My recent smaller project used pretty much the same working set up and didn't have the same problems. So I think if it was doing the same things at that point, the smaller project footprint meant it didn't hurt RAMpreviewing as much. Unfortunately I don't have an intermediate sized project to test with at the moment.