Agree with all above. Idle speed is controlled by air flow. If your throttle plate holds position (most likely) a varying vacuum leak could make it surge. As mentioned, a lean idle makes it erratic. Also, the spark controls can cause a "positive feedback" situation where as the engine speeds up, it gets more advance which makes it go faster, ... I think weak springs on the weights can cause this. Also, since you are idling at 1000 rpm, I suspect you have trouble idling - like a big cam and big carb. That could make it run better faster, causing the same positive feedback condition.