What controls idle speed on om606?

[Parrot of Doom]

As title. Mine has started drifting slightly, quite randomly. It can go up to 1500rpm. It will drop the instant I apply the brake.

I have a video of the tachometer while it's doing it, I'll upload it tomorrow when I'm on my laptop.

Also, does anyone know, on a duovalve which part tends to go - the solenoid, or the plunger? Because the right side is playing up, and rather than buy a new valve I'd rather experiment and replace the coil.