1998 Ford Windstar Hard to start van when engine is hot

Check the coolant sensor; it may have failed, making the car computer think it is constantly cold: enriching the fuel mixture.

A rich mixture is fine for cold start-ups, but when the engine gets warm, the air/fuel mixture needs to lean out. Without proper input from the coolant temp sensor, the mixture will stay rich, causing running trouble as you describe.

There's two sensors. One for the gauge needle, and one for the PCM.
Focus on the sensor with two wires: Should be LightGreen-RedStripe and Grey/RedStripe.

The other one with only one wire (Red/WhiteStripe) is for the instrument panel gauge.

Both sensors are next to each other near the throttle body on the intake plenum. Pull off the air intake snorkel and you should be able to see them.