Well, oiling up the running gear did not help me on Wednesday. However, this morning I tried adjusting the cable tension on the rear derailleur and that seems to have fixed it. At least, I was able to ride hard today without the chain jumping on me.
Also, and I don't think I mentioned it before, I broke my helmet the day I got it. I didn't realize it right away, but I was using the strap-tightening-knob incorrectly and I caused something to break; after a while I couldn't tighten it at all that way. Fortunately, there are several sets of mounting holes for the strap, and so I was able use those to keep it tight. Then, the strap came apart from the knob, so I took the whole contraption apart to see if I could fix it. I had some success, but in the process I stripped the threads in the knob so that I couldn't tighten it properly and any adjustment would require more screwing. This morning, the knob popped off completely; the threads had been stripped that badly. I got some handy gaffer's tape, made a tiny roll out of it, sticky-side-out, pushed that down in there, and threaded the screw into that. This seems to work pretty well. For the time being, at least.
Drafting an 18-wheeler, while effective, should not be attempted if you don't have eye protection of some sort. I learned this the hard way this morning. I got all kinds of dust and grit thrown in my face. I wish my computer was working, though, 'cause I had to be doing better than 35 that way.
Also, today's ride did not involve the lowest front sprocket at all. Minimum gear was 2-2. It should have been 2-3, but I stepped down one too many at the end of the last hill at my turn-around point. I think I may have to invest in a new front crankset; the low sprocket is almost useless and the high sprocket isn't high enough. My legs were flailing as fast as they could while drafting that truck down the hill.