1992 Chevy Lumina ENGINE IDLE SPEED VARIATION

THE ENGINE STARTS OK, BUT SOMETIMES RUNS FAST OR SLOW. WHEN CAR IS PUT INTO GEAR ENGINE DROPS VERY LOW IN IDLE THEN SPEEDS UP FAST. DRIVING PERFORMANCE WITH THROTTLE OPEN IS OK, BUT WHENEVER THROTTLE IS RELEASED THE ENGINE CONSTANTLY GOES UP AND DOWN IN SPEED. SOMETIMES WHEN ENGINE IS STARTED HOT IT WILL IDLE VERY LOW THEN RACE UP QUICKLY. THIS IS A VERY ANNOYING PROBLEM THAT ONLY SEEMS TO BE WHEN THROTTLE IS IN THE CLOSED POSITION. I CLEANED THE IDLE AIR CONTROL MOTOR AND ITS HOUSING, BUT IT MADE NO DIFFERENCE. I DONT KNOW ANY OTHER WAY TO EXPLAIN THIS PROBLEM, I JUST KNOW THAT THE IDLE SPEED IS ALL OVER THE PLACE! HOPE YOU CAN HELP.