I was eventually able to 'solve' this problem by breaking my complicated page
into several simpler ones. Even removing chunks of cf tags _after_ the query
helped. Somehow, it was related to the sheer amount of parsable crap on the
page. I could just remove code starting at the bottom of the page, line by
line, until, miraculously, the query ( in its isolated loop far above) started
inserting correctly. I'd then add the code back, or add unrelated code to the
end of the template, and the insert would bomb again. Indicates some kind of
parsing problem to my mind, but I don't have the time or know-how to analyze it
further. Perhaps this will provide someone else a clue to a seemingly
unrelated problem.