lake 99 chevy lumina idling extremely erratic?

Your throttle body probably looks like an used oil pan inside by now. Locate it and make sure that the throttle plate is closing correctly, with no restrictions. Then, locate and clean your idle air control valve, which is mounted TO the throttle body. Also, check for leaks in your PCV valve and hose. Any leak will cause your idle control computer to go nuts. Also, check your intake manifold for any signs of leaking, they have a tendency to with age. Good Luck!!there