I don't think it's an outside YAF event causing this. The YAF Installation Wizard works perfectly; also, I have another web application (on the same server) written in C# that has no issues with sending emails.
I did start deleteing the rows in the yaf_mail table. Not sure which record, but after 3-4 records, I deleted one and all the rows flushed and emails flowed.
I "suspect" there was a record in the table that caused the send mail method to fail. The "sporatic" actions, I'm guessing, are caused by the order of the sql query return (probably no ORDER BY in SELECT query). Sometimes a new email record would go into the table, change the order of rows returned by the query, and they'd be processed until the bad record was hit...then it'd hang again.
Sound reasonable?
Keith