Hi DSR, thanks for your reply.
The problem has toned down (mostly, now I only have the occasional keychain sync loop). To solve the problem, I deleted all signatures on in all copies of Mail, and then put in a new signature in only one copy of Mail. It then sync'ed fine.
I'm pretty sure that it was not network related.
I haven't really looked at sync'ing junk mail behaviours, but that's a pretty cool concept (and very helpful). As for the Mail hanging issue, it hasn't affected me, so I'm afraid I can't comment.
Cheers!
Chas