Your engine may have a vacuum leak or maybe the idle control valve is due for replacement.
replace idle control solenoid, check for dried out vacuum lines, pull each line off and look underneath on hot engine side which tend to split first. if rubber lines feels hard as a rock or split at end where hose plugs in, replace thjem all.
throttle position solenoid is a common problem on town cars to cause this problem, easy fix if u got patience, pain in the ass if u don't, located at back top of engine, ur tools will barely fit back there but u b very happy when u get the job done yourself. i have done a half dozen of these.so be ready to do the happy completion dance or U may be yelling :"What a stupid place to put the throttle position valve"!!
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