You have a vacuum leak or a leak of the piping which comprises the idle air system. It could also be an idle air control valve stuck open. Your car should idle at 650± 50 or thereabouts. Even 800 is a little high. Get it looked at soon since slamming the car into gear at 1100-2000 RPM will land up costing you much more in transmission repairs than correcting this relatively minor issue.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz