The only pattern I could establish was that emails after a certain date in one POP account only were all problematic (failed to move). When I rang the ISP who hosts that POP account they asked me to forward the full headers to see if there was any junk in the headers. They said sometimes that happens but they couldnb't find anything wrong with the headers. They said without a more specific error log they couldn't do anything.
It turns out they are finally offering IMAP on the server my account is hosted on and we switched from POP to IMAP which sorta helps, at least I have remote storage if I can recreate the folder structure on the IMPA account and have Mac Mail mirror it… I'm not too hopefully of that actually. Co-incidentally they added IMAP around the same time that I started getting this iCloud problem with emails on their account but they denie any assoication other than co-incidence and I couldn't estanblish exact chronological symetry anyhow.
My work around has been to host the folder “On My Mac” since I no longer need to share the folders b/w two Macs. I'll try to re-create in IMAP account like I said. If anyone has a way to get a more detailed error message from Mac Mail I'd be interested!