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On mine, the harmonic balancer was wobbling and made the car have a rough idle, but it also developed a vibration around 2400 rpm. Needless to say I swapped the balancer for a new one and it eliminated the problems. Not saying that's your problem but it was my personal experience.
Have you ever cleaned the MAF sensor? You can spray it with a sensor safe cleaner. I know they have a lot to do with idle quality and if its not reading right, that may be the problem. Just a guess?