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Hello and welcome to Just Answer. My name is James.
When you shift into gear, does the engine stop abruptly with a bump feel, or does it sputter and die?
Without seeing the vehicle myself, the most common cause for the concern you describe is a locked up torque converter clutch in the transmission. It's supposed to slip at idle in gear, but when the friction clutches inside fail, they can stay locked, stalling the engine. Do you know if trouble codes have been checked in the processor?
Okay, if they relay that information to you, it would be helpful. Otherwise there are a number of engine control related possibilities here. First off, 2003 was the last year for the Windstar. Is it a Freestar, or possibly a different model year?