SharePoint 2007 Alerts Issue

We have multiple SharePoint sites and many lists within each site. In all sites, a user can create an alert for themselves on a specific list. If they choose a daily summary alert or weekly alert and click OK, it appears to have been set correctly. However, the alerts come immediately and many are sent each day. When the user goes back into their alert settings, sure enough, it has been reset to immediate alert. Any ideas as to what may cause this would be appreciated.

Thank you

[497 byte] By [mpapier] at [2008-1-10]