do you have a 'check engine' light...and is it on?
you may have a vacuum leak, faulty AIC motor (or throttle switch).
pop the hood and listen for a sucking noise. (i love that).
an engine temp sender (for the injection) could be at fault, or as noted above, a low coolant condition will act the same. the system never knows that the coolant got hot, and keeps your idle high as if it was still cold.
if your CE light is on, i can tell you how to pull the codes.
a vacuum leak is the most common cause.
hth