After engine reaches operating temperature, RPMs increase while in gear. 6 cylinder, automatic. In PARK or NEUTRAL, idle is normal, when placed in gear, idle increases making it hard to hold car still at STOP signs or red lights.
Your engine probably has a bad idle air control valve or throttle body. If you see warning lights, take the car to an auto parts store and have associated codes read, probably free, then you'll know where the problem is.
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