Not good. Was my belt too loose? No, it strums nicely. Was there a piece of junk interfering with it? Well, there were two pieces of metal sticking out of it:
But that wasn't it, either. Just when I was about to despair, I noticed that the hotend was wobbling back and forth during a print. I should have known - it had once again come loose, probably when hitting the side of that piece that warped up. Alright, that's it, I'm switching over to the new extruder holder I printed earlier, the one that has nophead's sneakier mount system.
Fiddling around with that took a bit of time, but wasn't so horrible. I have to mount the hotend before putting the extruder on the carriage, but it's at least solidly mounted now. Also lots of nut traps.
When testing the mount, it turns out I've lost about 5 mm of build height - oh noes! Had to recalibrate X as my fiddling around had moved things. What's worse, and somewhat strange, is that even after setting a Z offset in Slic3r, it goes several mm too far down when the head goes into position to heat up. It appears to be ignoring that offset, I should probably make the firmware have the right limit instead. Not tonight, though.
You can see here how the new mount works: Three screws with washers (two visible here) hold the top of the hotend into the hole. As long as the washers hold up, this will be very solid - possibly solid enough that I should worry about what part would break instead if there was a bad head crash. The 5 mm less height comes from the mount having a much shallower hole, for not having to have space for screws under the top of the hotend.
I'm also concerned that the Z rods are noisier and more wobbly than earlier.