don't know what you mean by "idles up and down" but I know my 88 aries has an idle problem. I took it to one mechanic and they said most likely a vacuum leak....but wanted to charge me 80$ to "diagnose" it.....so I've just been driving it.
It runs GOOD once it's in Drive, but it has to warm up a little bit, with a little feathering of the gas pedal to keep it going. Once the gas is flowing good, it quiets out, and can safely be engaged into Drive, and off it goes without a problem
When I get to my destination (parking space, for example), and switch thru the gears, the engine gets slightly louder in Neutral and in Park......like it's idling high.
I have a feeling it may be the MAP sensor vacuum line......I previously had a MAP code, so we replaced the MAP sensor (easy replacement btw....heheh, just 3 screws, drop it in, and reconnect the vacuum line) the MAP code is gone now but the high idle still occurs.
What I've been doing lately is injecting starting fluid in to the Throttle body in the morning, 3 short bursts, and it helps the car start up a little bit better.