Why does an '89 Honda Civic's engine sometimes start racing on its own while driving?

Its cold, idling, and old.

3b7From my understanding with my own 89 Honda civic LX sedan, is that there is a thermometer that sets idle speed at low and high temperatures, with age, ours have degraded, and the low temp. has been borked. It doesn't hurt your car any, I've checked with a few sources, it just means while your engine is warming up in winter, your car will do that little rev thing every few seconds while idling.