1.8 Petrol Engine With Variable Idle Speed

[overthetop]

My own 2008 Avensis 1.8 petrol engine has its own idea of what is idle speed.

Typically, the engine is very slow to come down to 700 rpm when I stop at traffic lights.

Frequently it hasn't reached correct idle speed before the lights have changed back to green.

Whilst at the lights the engine speed occassionally actual goes UP during its very slow decent.

No, it isn't one of the recall models!

Toyota garage say they can find nothing wrong using OBD equipment.

Any ideas?