Some of these models are fitted with an fast idle sensor which when it
senses extra load automatically ups the revs. Typically you can test it by
turning on your AC or pushing the brake hard and turning the steering wheel.
Sounds like yours is starting to fail.
> I have a '93, 605 three litre that has variable idle speed, sometimes idling
> at 2000rpm when hot or not slowing from fast idle after starting. Any
> suggestions please.