Finally had a chance to have another look at this issue and did what I said I'd try above. With a completely cold engine, first I unplugged the exhaust side ignition coil and fired up the car. It started pretty quickly as usual but fumbled around at a low idle speed for a few moments before zipping up to 2000 rpm where it ran smoothly. Interesting, says I. So I shut it off, reconnected the exhaust coil and disconnected the intake coil then restarted. She fired straight away and zipped right up to 2000 rpm where once again it ran smoothly so I took her around the block to warm the engine expecting the idle to drop back down. With the engine warmed the idle remained at 2000. I shut it off and reconnected the intake coil, started it up warm again at 2000 rpm.
I had brought the idle speed up when it first began stumbling so now with a warmed engine I brought it back down to about 800 rpm and took her out on the highway. Zoom Zoom. Mazda's got nothing on Nissan. She ran absolutely perfectly.
The only thing I noticed was a quick whiff of a burned oil smell just after the idle jumped to 2000 rpm the first time. Since then there's been no such smell. So that's got me wondering if some hunk of greasy crud was lodged somewhere that finally got blasted into gaseous oblivion. I've still got a new cap and rotor on the way that I'll just hold in reserve.