my dad's astro used to have a similar problem...just sitting in park it would idle high then low then go back high then back low then high then low then go back high then back low then high then low then go back high then back low then high then low then go back high then back low then ....u get the idea....it turned out to be a vaccume leak...that might be a factor in urs...not positive though...u might have a garage chk it out though