Sorry for the Long Radio Silence...
But finally I've shaken free from the writing-related miasma. What did I do? Gave into the inevitable and put the story into first person. I'd been struggling to write in third person, but my natural writing style seems to be first, and once I let myself write in first person, the words began to flow again.
So I guess the rule is, give up on third person, at least for now. Go with what feels most natural, most comfortable, and just let the character drive.
