Have this car. Had this problem, Drove me crazy for a year. Remembered the same issue with my Lincoln. I cleaned the idle bleed port with carb cleaner...really cleaned it...easy to do. Take the intake assembly off...take out the screen covering the intake throat and use the extendor tube that comes with the cleaner. Spray directly into the port above the butterfly. Worked perfectly. Never had a problem since. Might not be your problem but it is an easy place to start.
Brian