modelbuilder crashing

Hi

I am building a model perform several summary statistics on a dataset, I need it grouped by different casefields. However it recently (in the last two days) has been crashing when I press save (not when running the model-just when editing it). I am prompted that it can't save and then the model closes and an x appears in ArcCatalog over the model and it is empty when I open it.

I am thinking that maybe I have reached a limit in modelbuilder, does anyone know if this is possible or have any other ideas???
-the models are quite large and I am on my 3rd redo now. Strangely, I have been working on the model for a while now, and it is only recently that this has started to happen and there haven't been major changes to the number of tools in the model.

thanks for any help,

Sharyn