I hadn't looked at the plugs since putting them in, so today I figured I'd try that. If you think changing the front plugs on a 2.8 (or 3.4) is hard, that's nothing compared to a 3800. The only way I can reach the plugs is from underneath, and there isn't much working room.
So I jacked up the rear and put it on jack stands and dove in. And there I found the problem. The wire to plug no. 3 was not on the plug, but hanging down. I don't know how that happened, except I may not have pushed the wire on enough. But I pulled that plug anyway. It looked new except a little wet as would be expected. I put in a new one anyway, and made sure all plug wires were on tight.
Then I fired it up. It runs smooth now. Still a little tap-tap (bad lifter?), but no missing.
I may just have to buy this thing (at a big discount due to all the parts and sweat equity I've put in it -- hint, hint), and tag it so I can take it on a decent test drive.
I don't know why, but I was under the impression that it was a Formula. I ran the VIN, and it's a base coupe. Quite an upgrade for a base coupe.