90 integra, surges at 3000

I just picked up a 90 integra 1.8. When I first start it, the idle is at 3000 then it goes to about 1500 then surges down to 1000, but if I turn on the a/c it idles at 1000 with out surge, Can someone offer advise. This car is just to save me on gas so I dont want to spend a ton of money fixing it...:banghead:

you might have a dirty sticking idle air control valve and or dirty air filter