Elantra 2000 acceleration problem

I have the following problem with my elantra since many months. When i'm driving and i try to accelerate rapidly the motor stumble, but most
of the time it do this the first time for the subsequent the acceleration is normal. Recently it started to stumble at idle, today the car took many trial to start. So it's seems the problem get worse.
Sombody have an idea? How to check for the problem?
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