We are aware of some potential issues with not releasing certain memory after renders. That may be what you're experiencing. But it seems odd that it would be affected by crop rendering. Are you certain that it is *worse* with crop rendering on a per-frame basis, or is it possible that you're just able to render more frames in a shorter time, and so the issue shows up sooner?
One possibility for working around this would be to use an external render manager that calls Terragen from the commandline and thus restarts TG each time, clearing the memory automatically.
- Oshyan