My car engine idle is below 500rpm, what may cause this or how can I fix it?

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Two months ago I replaced the spark plugs in my car. I was told the spark plugs did not need to be calibrated. Two-three weeks later, I've noticed the car starts and idles +/-@ 2300rpm for a couple minutes then it drops below 300rpm and it shuts off. I have to start the car over with my left foot on the brake and my right foot on the gas and ride like that. Did the spark plugs cause this?