Year? Engine? Transmission? Modifications? Odometer? Driving conditions/weather/etc.?
Without any more info, I would venture to guess that your Idle Air Control (IAC) valve is sticking. Usually, a good cleaning is all that is needed. If that doesn't do it, it is a reasonably easy (5-30 minutes) and cheap (<$50) replacement job.
1999 Contour SE Sport MTX75 T-Red/Grey
Audio: Pioneer DEH-7400MP (MP3 CD), Pioneer TS-A6857 front & rear, PPI PCX-250, Infinity Perfect 10.1
Other: cup holders, cargo net, Blizzaks, Potenza RE950s
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