I was taking my eldest to the horse farm where she rides today when my car's engine started running weirdly. It felt like a stuck throttle which I had experienced once in my 1984 VW Scirocco, but on further diagnosis it was not; the throttle was closing just fine. The engine would hold about 3000 rpm after being revved and releasing the throttle, then slowly creep back down to around 1500-2000 rpm. This was not good for driving as it had sufficient power to hold 50 mph with no throttle input and required extra brake pedal pressure to hold it stationary at lights; completely controllable with brakes, but not good. So we turned around and came home.
Subsequently, a bit of research and some diagnostics suggested the Idle Air Valve which is an adjustable bypass around the throttle plate for fine control of the idle speed.
A quick trip to Autozone, $58 + change, 5 minutes to R&R the faulty one and now the car is running fine again.