Dear wwh: I thought of Plutarch, too, in this context--but it doesn't quite fit here. What is the second word of a hyphenated couple called? You know: okey-dokey. Is there a word for that second part? Maybe it could be fused into Plutarch's omegatist.

Dear tsuwm: It's endearing if I can't pull out the part of the sentence I mean to say; it's bothersome as the devil if it's a person who simply enjoys staying one step ahead of me, which ain't too hard to do as I make the transition between brain-dreamland to understandable human conversation.

syntaxfinisher?
syntafinisher
syntaomegatist
syntaxomegatist
pain in the butt? (butt taken here to mean the end of the thing when you're aggravated by the interruption, of course)

Butt regards,
DubDub, the very patient listener, but willing to offer a helping final flourish, if you need it