I have an idle problem that's been bugging me for a while now. When the engine is hot, and the car is stationary and in N or P, the idle will slowly rise to about 1500rpm and stay there.

It certainly isn't a problem with the cable or the idle setting, as when cold the engine is about 500-600rpm.

So I'm thinking something in here is getting a bit toasty, or perhaps is going near-open-circuit when hot. Any pointers, other than a faulty throttle pot (too expensive to guess at)?