Found a "Fix"
I went to the Apple forum and had to sign in, so lost the link you had given..... however in reading some of the posts there about this problem, I found this. It worked. God bless "cdown."
Re: MAC Mail not saving all sent items
Feb 22, 2013 4:29 PM (in response to chrisaxe21)
I was having the same issue...random messages not appearing in my Sent box. I figured out that the Sent box was sorted differently than my other mailboxes. It was sorted by Unread Descending. So, to correct the problem, while I was in the Sent mailbox, I went to View, Sort By, and selected Date and Descending and all my problems have disappeared.