Idling speed went to "zero" and engine quited

I am a poor student and own an old 1995 Ford explorer(134K miles,auto transmission). Recently,I did oil change and then a new problem appeared. when I started my car, the engine speed gauge showed around 1500 rpm with the noise. But when I loosed the key, the engine speed was suddenly shown a sharp jump down to below 300 and the engine sound also almost disappeared. Mostly, the engine quited working and sometimes the idling speed automatically jumped up to over 800 rpm in half second. And I could feel the shaking of the car at “P” gear. I searched on website about this issue.someone thinks it is because the idle air control motor failed to work? what do you expert think about my situatioin? How much if I have to replace it(part? labor?) Please help me, I really expect and appreciate any advice from you.