Help! Calendar app quits unexpectedly

On two of my Macs (a MacBook Pro and a Mac Pro, both running Mountain Lion), the calendar app quits unexpectedly on a surprisingly frequent basis. It works fine for entering one new calendar entry. On the second or third entry, whether adding a new one or editing an existing one, it will always quit. Before quitting, the screen freezes with the beach ball of death. Then, bam! Calendar is gone. After months of this, I'm extremely frustrated.

I have tried deleting the iCal preferences and it hasn't helped. I'm running 10.8.5 now but the problem started several versions ago.

Any suggestions as to what I could try to fix this?