comp disappears in custom view cam

hi all

I'm having a weird problem viewing my 3D comp in the "custom view" camera;

everything is fine until, at a specific frame on the timeline, my 3D comp simply disappears in the custom view window (i.e. I toggle back and forth between these two frames, and before it's there, after it's not). It's still visible in the "active camera" and other views--and I can't find any keyframes at that point that might be shutting it off, or otherwise obscuring it.

Plus it's just happening in custom view....

I'm using CS5.5 on Windows 7

Any ideas?

thanks

mL