Well, I've got some news on the car. I talked to the service tech, and he said it did act up for them (finally!). There's something called an IAC valve (intake air control, maybe?), and he said it's sticking full open, he thinks it's trying to compensate for the engine getting too much fuel. If it's just that the valve is bad, it won't be too terrible to fix. If it's something more complicated, I don't know how bad it will be to fix. Part of me is glad they're making progress, and part of me is vaguely nauseous at the thought of how much it might cost to fix.
But I knew it was the fuel system! I should be a mechanic.