idleing issue

I have a '92 mx3 auto w/4cyl. It idles @ 900rpm in park and
Neutal. When I put it in any gear it runs rough and the rpm's bounce from 900-300. The only way I've been able to get it to run smoothe in gear is to turn the air conditioner on
and turn the heater fan on, then it kicks the rpms up to 900 and idles smooth then I turn of the a/c or the fan and goes back to running rough. Any thoughts?
Thank you, Jon.