Some thumbnails not loading

Hi, I've installed BG on several sites and overall I think it's fantastic. The main issue I'm having is that one of my clients has galleries with several hundred photos; this seems to be causing some of the thumbnails not to load (ie, broken image icons). The actual full-size Lightbox2 images load properly.

My question is: Is this due to an issue with BG, is it an issue with the memory allocations of the server (ie, PHP is limited to 64MB as opposed to the 128 or 256 that are recommended), or is it just a matter of it not being reasonable to expect the server to load up to 600 thumbnails on a single page at once?

Suggestions appreciated, thanks!