1985 Pontiac Fiero High Idle Speed

Recently, when you turn on my car it starts out idling anywhere between 1500 rpm to 2000 rpm, which is somewhat normal for startup. But if you let the car heat up and you start driving it all of the sudden, it will start accelerating on its own. So I take it out of gear and the car will be idling somewhere between 2600 (highest) and 1300 (lowest while acting up). But if you come to a complete stop, the idle eventually goes back down to 900 and will stay there until you accelerate again. The check engine codes are Vehicle Speed Sensor, EGR Circuit, and Idle Speed Error. I've seen a lot of cars diagnosed with the problem of mass air flow sensor, but this car doesn't have one. It is also fuel injected. One more piece of important information: whenever you disconnect the vehicle speed sensor, the car gets lower gas mileage and feels sluggish but it also doesn't idle higher. I tried replacing that but nothing happened. I've looked into the throttle position sensor, but this problem looks more like a vacuum leak. Can anyone offer me any help? Thanks