Well, I just found the problem.

I checked the distributor, and the lowest plug wire was free. Sorry for not checking that earlier before posting.
Looks like the last mechanic to work on it didn't attach the wire all the way (I had some work done on it recently). Gotta say it's a chore to get that lowest one on. But the insulation is charred on the tip so I'll need new wires. We'll see what the mechanic says about replacing the wires.

No wonder the car ran worse in the rain, you can imagine what the rain did to an open plug wire.

But mostly, I'm just surprised how well the car ran on only 4 cylinders!!