When the throttle is closed at high rpm - like descending a hill or leaving the highway - the valve opens under the very low pressure created in the intake manifold, and admits enough air to prevent misfires. As the engine rpm drops to idle, the manifold pressure rises a bit and the valve should close. Of course, over time the spring weakens, and it's capable of then opening at normal idle pressures, causing the fast idle.
You might also check the the distributor advance weight springs are pulling the weights in at idle. It's tough to determine, but sticky weight pivots or weak springs will allow the advance to "hang up" and that will also cause the fast idle.