The fact that it was idling on 3 cyl when warm but ran smoothly under load convinced me that I had a vacuum leak somewhere. It didn't act like a leaking injector because it started hitting on all four immediately when I hit the gas.

I tried the unlit propane torch to look for the leak, but the car was running so badly that the results were inconclusive.

I was 'sure' that the problem could not be the intake manifold gasket, since I just replaced it a year ago, and because I snugged down the manifold nuts and did not get any appreciable change in behavior. However, I was determined to leave no stone unturned, so I dismantled everything all the way down to the head.

That gasket was a mess. Worst I have ever seen. It had crept into the #1 and #2 channels and was partially blocking them, and it was just disintegrated around #4 and was leaking like a sieve. I should have taken a picture.

I replaced the vacuum hose to the heater control, but all of the other vacuum systems checked good. I also poured some Seafoam down the EGR tube and let it soak overnight before starting the car.