You need to provide more info on your settings, what footage you use and your system hardware... Disappearing RAM previews usually occur when using MP rendering on an unsuitable system or with specific CoDecs that flake out...
.MOV's straight from a Canon T2i.
Previously these files did not do this.
i7, 12 GB, GTX 480, latest driver,
Now it has stopped behaving this way. No settings in AE were changed.
Can't reproduce the phenomena.
I must confess, I've noticed this behaviour with almost regularity lately, on several different systems. Not quite consistent enough to prompt a bug report (or thread here) but I have definitely seen what the OP is describing.
Originally, I thought it was being caused by VCP Optical Flares plugin, but since then have seen it happen in projects without the plugin in use.
I've also seen it in both CS4 AND CS5, which got me wondering if an OS X update may have done something to memory management or something.
I'll keep an eye on it and post back if I can find anything consistent enough to report.
Can you track the behavior down to any recent changes?