My car when I idle its runs fine when its been a while running I idle normally and it goes blank,…

My car when I idle its runs fine when its been a while running I idle normally and it goes blank, like it revs and it slow down the car even when i press the gas. When i press it hard then it catches up. And starts running good but it does it alot of times. I tigthen the idle cable, and the revolutions on Parking they are weak

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