While I am far from a subject matter expert on the topic of lock cylinders for Ford Windstars (or any other sort of lock cylinder you'd care to name), I will offer a Scientifically Educated, Wild A** guess and say that the lock cylinder in the ignition of your Windstar is so severely worn that it no longer has the integrity to function correctly (sort of like.....Oh, I don't know...Could it be perhaps, THE UNITED STATES GOVERNMENT!?!?)
As an aside, for those of you not fortunate enough to have seen Dana Carvey as "The Church Lady" on SNL, you won't get the humor (although I hope you can perceive the truth).
Back on topic now, Tom. If I were you and had a friend who was super sharp as an amateur mechanic, you might ask them about the issue you've described. Otherwise, I'd be inclined to learn the process for replacing the ignition lock cylinder in a Windstar ( there are an abundance of resources to direct you). The cost of the part itself varies widely from less than 50.00 to more than 250.00. If you decide to go this way, carefully shop on E-Bay because I have picked up some unbelievable bargains there (in the interest of full disclosure, I have ended up with a 'clinker' or two as well)
Best of luck, my friend!