1999 Ford Windstar why?

Check the IAC Valve. It's mounted on top of the throttle body, and comes off with two 8mm bolts and a wire connector. IAC valves (Ford) are chronic for going bad, clogging with carbon and/or blow-by engine oil. Then they stick, causing improper idle.

You can try cleaning it, but I suggest replacement.

While you're at it, take off the throttle body and lightly clean the inside of blow-by oil using a good solvent. There may be a label indicating "do not use chemicals for inside throttle body cleaning", but I've cleaned plenty of these using brake cleaner without any ill transient effects. Blow the chambers out with compressed air, make sure all the vacuum ports are clear & re-install.

Good luck.