2001 Hyundai Sonata Idles Rough/Shuts Off Intermittently

Hi! My 2001 Hyundai Sonata has been idling rough/shutting off intermittently for the last 6-8 months, with the "Check Engine" light coming on only about 2-3 months ago. I have tried cleaning the fuel lines, replacing the IAC motor, and, today, replacing the MAF sensor. It seemed like the problem was fixed after changing the sensor, but it started to idle rough again. (The car will idle @ about 800-1000 RPMs, drop down to about 400-600 and then shoot back up again. Occasionally, after driving for long distances and coming to a stop @ a light, the car will just shut off).

I took the car to my local service station to see why the "Check Engine" light was still on, but there were no pending codes. (The light is off now.) Since this morning, I have changed my air filter and tightened the air ducts, but the car is still idling roughly every so often. What am I missing here? Please help! I can't keep dropping money on parts that are not fixing the problem.