Since I added octane boost and then started using premium gas, my car is running great again. So insufficient octane was the problem. Crazy.
Since then, several other sources have confirmed that modern cars do this, though almost nobody I know had heard of knock sensors. And today I ran across an article that describes exactly what happened, and even mentions my car specfically.
High-octane gas isn't just for tuners though. Plenty of stock cars depend on the stuff, including a Celica GT-S with its 11.5:1 compression, or a turbocharged WRX or Volkswagen 1.8T.
What more, my car's manual says they recommend 95 octane, which isn't even sold at gas stations. (In California they only go up to 91 these days.) So the question of the day is: if I boost the gas up to 95 octane, will I notice a difference? A couple months ago I'd have said "no way". Today, I'm thinking about trying it.