MOSS 2007 Alerts - Multiple Emails Issue

I've been experiencing a strange issue with SharePoint alerts that seem to happen randomly.  I've setup a daily alert on a custom list that sends a summary of all changes at 5pm, but on some days I'll receive more than 1 summary email (150+ this past Tuesday).  Each email contains different information and the total number of emails is random along with the number of records in each email.  Some days the summary comes in one email and others it comes in multiple.  This is happening on our production server and I haven't been able to recreate the issue in development.  Does anyone have any thoughts as to how a daily alert would get split up into several emails?

We have 1 MOSS 2007 app server, 1 index server and 1 SQL 2005 database server.