I just happened to remember one thing that did change around the time that this new symptom cropped up.

I just installed a brand new cast-metal valve-cover (looks cool!). While I was doing it I noticed that there was a small tube running from the exhaust manifold (i think that's what I remeber away from the car now) to the distributor body somewhere near the bottom of it. Is this some kind of advance return suction system? I noticed that the new valve-cover is somewhat taller than the old pressed type, and this tube was not clearing very well over the top of the cover.

I remember feeling slightly worried that it was pulling too hard or even kinking. Could this effect and cause the symptom I described? This tubing seemed kind of stiff and different from normal fuel line type rubber stuff, so I was worried it would be a hassle to track down a new longer length of it.