It might be your Ram, if your modeling and you've not deleted the histroy this can build up and cause slowdowns and then crashes.

Mac's when pushed to the limits of their Ram have a tendancy to fall over.

Try deleting the history on your model and then see if it happens.

I think that multicore processing only came out with maya 2008.

Then again it might be what Chris said, the PLE is buggy on a Mac.