It seems that I work by driving toward and building connections between key scenes, using the next upcoming juicy scene as bait to drag myself forward. I'm slightly stalled because I've run out of juicy bits to tempt myself with. So I wrote down a whole bunch of potential juicy scenes for the second half. Some of them are from the original outline, some are brand new. There are 32 of them, which is too much stuff for one half-novel even if many weren't mutually exclusive, so some will be discarded and/or combined with others. I printed them out and cut the printouts up so I have 32 little rectangles of paper, with the intention of shuffling them about until they seem vaguely plot-like and then dividing them into chapters and viewpoints. This is an exercise recommended by Tim Powers and others, which I haven't done since my original outline for novel #1 (the one with the insanely complex braided timeline, for which the outline eventually became a color-coded spreadsheet).
In the process of preparing the 32 little rectangles I discarded the original ending (it always was a bit of a downer) and replaced it with a new one suggested by something lrcutter said. It's uniquely science-fictional, makes good use of a key element of the setting, and resolves a key ethical dilemma all in one tidy package. Now all I need is to find the way there. (I always know the ending of a story... generally it's the second thing I know, after the setting or situation. Sometimes the end changes as I move toward it, but I always have an ending that I'm heading for.)
This evening I realized that I have to get this chapter done by Saturday, and lots of other things too (including the season premiere of Dr Who on the Sci-Fi channel). I also realized that even if I don't know exactly what's coming next I have a fair idea of the rest of this chapter, so I started drafting again. It's going to be a stretch to get it done. I'll finish re-outlining after that.
PSA: If you are one of the eight people who was watching Drive when Fox unceremoniously canned it after four episodes, you might want to know that the remaining two episodes will be aired on Friday July 13 (not July 4, as originally announced).