Matrixfan88

Hello everyone, I am new to this forum but as far as I was told, this forum could help me understand what is going on with my vehicle I am currently driving. When I start my vehicle up, it idles very low, almost to the point of cutting off, and then it idles very high for about 3 seconds. After it idles high, it levels out the idling. The mileage of my vehicle is 242,784, I didn't know if that made a different of why it would do that strange idling. It does not happen very often but when it does I kind of get worried of what is going on. Does anyone know what might be the cause or causes? I really appreciate it.