My NSX usually idles at about 1000rpm when warmed up.

Recently it has started to sometimes idle at higher rpm even when warm, at approx 1300rpm. There doesn't seem to be any adverse effect from this, but I'm curious as to why it might be doing it.

Interestingly, if it is idling at 1300rpm and I switch on the headlights, the idle speed drops back to 1000rpm and stays there, even after switching off the headlights. However, if I then raise the revs by blipping the throttle the idle rpm settles back to 1300rpm again.

Any thoughts?