Try cleaning the throttle body. If that doesn't work, there is a sensor
inside the intake manifold that may be dirty and need cleaned. If you pull
the air cleaner and associated plastic piping you can just barely see it
with a flashlight.
Spray throttle body cleaner in there really good and clean it with a lint
free cloth, or just have it replaced.....bout $200 at a shop.
Joe

pretty close, but actually a little safer for the man-made components. Just
make sure the engine is not running as you initially spray it in the
throttle body. Use a good amount then look for the sensor inside the
manifold area; this is where you want to direct the next part. Start the
engine.  *** Don't get over enthusiastic with this part*****Then only use
very short bursts. You'll notice the engine stall and black smoke will come
out the tailpipe. If you spray it full out you can damage the O2 sensors.
The cheapest O2 sensor is $73, so short bursts are in order. Wait for the
engine to smooth out between shots. After a dozen or so shots to the
throttle body with the engine running, shut it down and look for the sensor
I told you about earlier. It should look clean. If not you can clean it some
more or take it to a shop......
Hope this helps,
Joe

If you have the time and a torque wrench, you can do the job. You'll have to
pull the intake manifold to get to the sensor, so a Saturday is in order.
It's pretty straight forward, just get a Haynes manual and "have fun". Most
likely just cleaning the throttle body really good will solve your problem
though.
Joe