02 Sensor??

Whenever i have been driving my car for about 15-20 minutes and the engine has gotten the warmest that it gets, whenever i pull up to a stop light, and put in first with the clutch in (or in neutral) my engine will rev up about 500 rpms and drop down to idle and keep doing that until i take off or rev it up. My best guess to this is an 02 sensor, or a disconnected vacuum hose, what do you guys think? It's a 92 prelude si.