I am checking in the log files more extensively now that I had a specific time and user that the site failed on. I see a GET request from the brower to this path:

/stores/StoreOperations/Holiday2007/tabid/176/spacer.gif   
And this request returned a 404 status code (which as we all know is a Not found status)

This file path doesn't actually exist. It is more like /stores/website/portals/0/resources/StoreOperations....
Is the site not constructing the paths correctly? Is there something I can check or fix? The site does not break constantly... but sometimes on a daily basis.

ALSO, in the Event Viewer on my DNN portal I see this error:
InnerException: The SMTP host was not specified.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Net.Mail.SmtpClient.CheckHostAndPort
StackTrace:
Message: System.InvalidOperationException: The SMTP host was not specified. at System.Net.Mail.SmtpClient.CheckHostAndPort() at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, String Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)


This makes me think I haven't set up an SMTP host or the mail host is failing. I haven't seen this error at every site failure, but maybe it can help find what IS causing it. 
Thanks!!