Why does your 1986 rx7 idle up and down when you first start it?

The car is designed this way for cold starts. It should idle up to 3000 rpm then makes it's way down to around 1500 and finally down to 750 as it warms up. DO NOT drive when it is cold or If you have to do not take it over 3 grand until warmed up or you risk blowing coolant seals.