how much ram do you have on your system? Don't leave many open sequences, as they eat up RAM. Next, do some house keeping by making copies of your project files and then deleting unrequired sequences in your project.
Missed the obvious one. Out Of Memory errors can tell you you're running out of hard drive space. OS-X treats hard drives as extensions of physical memory.
Always maintain at least 10% free space on all production machine drives.
Thanks for the assist. It seems the problem may be something else entirely. I have plenty of drive space, and rebooting and re-starting the computer didn't help, so I started the project over and have not encountered the problem again.